Mentioning some of the evidence al-Bukhari cited regarding the obligation to act upon the report of a single narrator, and likewise the argumentation using the words of Imam al-Shafi'i in "al-Risalah"

The Shaykh : Then Imam al-Bukhari cited hadiths, arguing with them for what he mentioned regarding the permissibility of the report of a single narrator. The intended meaning is the permissibility of acting upon it and the statement that it is a proof. So we will cite some of them:
The first: From Malik ibn Huwayrith, who said: We came to the Prophet (peace and blessings be upon him) and we were young men of similar age. We stayed with him for about twenty nights. The Messenger of Allah (peace and blessings be upon him) was merciful and gentle. When he thought that we had become homesick or missed our families, he asked us whom we had left behind. We informed him. He said: . So the Prophet (peace and blessings be upon him) ordered each of these young men to teach his family. Teaching encompasses creed, and indeed creed is the first thing included in this generality, as mentioned previously. If the report of a single narrator did not constitute a proof, this order would have no meaning.
The second: From Anas ibn Malik, that the people of Yemen came to the Messenger of Allah (peace and blessings be upon him) and said: Send with us a man who will teach us the Sunnah and the Book. He said: So he took the hand of Abu 'Ubaydah and said: . Muslim narrated it, and al-Bukhari narrated it in a shortened form.
I said: If the report of a single narrator did not constitute a proof, the Prophet (peace and blessings be upon him) would not have sent Abu 'Ubaydah to them alone. The same is said regarding his (peace and blessings be upon him) sending of other Companions (may Allah be pleased with them) to them at different times or to scattered lands among them, such as 'Ali ibn Abi Talib, Mu'adh ibn Jabal, and Abu Musa al-Ash'ari. Their hadiths are in "al-Sahihayn" and others. There is no harm in the fact that these men taught those they were sent to the creed as part of what they taught them. If the proof were not established by them over those people, the Messenger of Allah (peace and blessings be upon him) would not have sent them individually; for that would be a futile act from which the Messenger of Allah (peace and blessings be upon him) is free. This is the meaning of the statement of Imam al-Shafi'i in "al-Risalah": "And he (peace and blessings be upon him) does not send with his command except when the proof for those sent to and upon them is established by accepting his report from the Messenger of Allah (peace and blessings be upon him). And he was able to send to them someone who would resemble them, or send to them a number, but he sent one whom they knew to be truthful."
The third: From 'Abdullah ibn 'Umar, who said: While the people were at Quba' in the Fajr prayer, a messenger came to them and said: The Messenger of Allah (peace and blessings be upon him) has been revealed to him tonight the Qur'an, and he has been ordered to face the Ka'bah, so face it. Their faces had been toward the Levant, so they turned to the Ka'bah. Al-Bukhari and Muslim narrated it. This is a text indicating that the Companions (may Allah be pleased with them) accepted the report of a single narrator in the very matter that was certain to them regarding the obligation of facing the House of the Sacred Sanctuary. So they left that and faced the Ka'bah based on a report. If it were not a proof to them, they would not have contradicted what was certain to them regarding the first qiblah. Ibn al-Qayyim said: "And the Messenger of Allah (peace and blessings be upon him) did not disapprove of them; rather, they were thanked for it."
The fourth: From Sa'id ibn Jarir, who said: I said to Ibn 'Abbas: Nu'fayl al-Bakali claims that Musa, the companion of al-Khidr, is not Musa ibn Israel. He said: Ibn 'Abbas said: The enemy of Allah lies. Abiyy ibn Ka'b informed me, saying: The Messenger of Allah (peace and blessings be upon him) delivered a sermon to us, then mentioned the hadith of Musa and al-Khidr with something indicating that Musa (peace be upon him) is the companion of al-Khidr. The two Shaykhs narrated it in full, and al-Shafi'i narrated it in this shortened form and said: "So Ibn 'Abbas, with his jurisprudence and scrupulousness, affirms the report of Abiyy ibn Ka'b from the Messenger of Allah (peace and blessings be upon him) until he declares a man among the Muslims a liar; because Abiyy ibn Ka'b narrated to him from the Messenger of Allah (peace and blessings be upon him) what indicates that Musa ibn Israel is the companion of al-Khidr."
I said: And this statement of Imam al-Shafi'i (may Allah have mercy on him) is evidence that he does not see a distinction between creed and action in arguing with the report of a single narrator; because the fact that Musa (peace be upon him) is the companion of al-Khidr (peace be upon him) is a scientific issue and not a practical ruling, as is clear. This is supported by the fact that the Imam (may Allah be pleased with him) established an important chapter in "al-Risalah" under the title: "The Proof in Affirming the Report of a Single Narrator," and cited under it many proofs from the Book and the Sunnah from page one hundred and four to page fifty-six after four hundred. These are absolute or general proofs that, by their absoluteness and generality, include that the report of a single narrator is a proof in creed as well. And his speech on them is also general. He concluded this research by saying: "And in affirming the report of a single narrator, there are hadiths, some of which are sufficient. And the path of our Salaf and the generations after them, to whom we have witnessed, has been this path. And likewise it was related to us from those from whom it was related to us from the people of knowledge in the lands." And this is also general. And likewise his statement on page fifty-seven after four hundred: "If it were permissible for anyone among the people to say that all the Muslims, old and new, have agreed upon affirming the report of a single narrator and adhering to it, and that it is not known that any one of the Muslim jurists has not affirmed it, then it would be permissible for me. But I say: I have not memorized from the Muslim jurists that they differed in affirming the report of a single narrator." And this is from his scrupulousness and precision (may Allah have mercy on him) in expressing his intent with precise scientific expression.
In summary, the proofs of the Book and the Sunnah, the practice of the Companions, and the statements of the scholars indicate with decisive clarity what we have explained; which is the obligation to act upon the hadiths of single narrators in all chapters of the Shari'ah; whether in matters of belief or practice. And distinguishing between them is a religious innovation that the Salaf did not know. Therefore, the scholar Ibn al-Qayyim (may Allah be pleased with him) said: "And this distinction is false by the consensus of the Ummah; for they have not ceased to argue with these narrative, scientific hadiths—meaning creed—just as they argue with them in practical, obligatory matters. Especially since practical rulings include the report about Allah that He legislated such and such, and obligated it and accepted it as religion. So His legislation and His religion refer to His names and attributes. And the Companions, the Tabi'un, those who followed them, and the people of hadith and the Sunnah have not ceased to argue with these reports in matters of attributes, divine decree, names, and rulings. And it has not been transmitted from any of them, ever, that he permitted arguing with them in matters of rulings without the reports, without the reporting about Allah... Practice is two things: knowledge and action. And what is required of the scientific matters is also knowledge and action; which is the love of the heart and its hatred, and its love for the truth that they indicated and included, and its hatred for the falsehood that contradicts them. So action is not limited to the action of the limbs, but the actions of the hearts are the origin of the action of the limbs, and the actions of the limbs are a consequence. So every scientific issue is followed by the faith of the heart and its affirmation and its love, and that is action, indeed it is the origin of action."