The Shaykh mentioned some of the evidence from the Qur'an regarding the obligation to accept the report of a single narrator, particularly in matters of creed.

The Shaykh : And the explanation of how it indicates the obligation to accept the report of a single narrator in matters of creed is as follows. First: The saying of Allah the Exalted: . Allah, Blessed and Exalted is He, has warned the believers that a group from among them should go to the Prophet (peace and blessings be upon him) to learn their religion from him and to gain understanding in it. There is no doubt that this is not limited to what is called branches and rulings, but rather, it is established that the teacher and the student begin with what is most important, teaching and learning the most important matters first.
And there is no doubt that creed is more important than rulings. For this reason, those who claim that creed is not established by the reports of single narrators are refuted by this... the noble verse. For just as Allah encouraged the group in it to learn and gain understanding in creed and rulings, He encouraged them to warn their people when they return to them with what they learned of creed and rulings. The word "group" (ta'ifah) in the language of the Arabs applies to one person and above. So if the proof were not established by the reports of single narrators in creed and rulings, Allah would not have encouraged the group to warn, explaining this by His saying: , which is explicit that knowledge is obtained by the warning of the group. It is like His saying in His legislative and cosmic verses: . So the verse is explicit that the report of a single narrator is a proof in... creed and rulings.
Second: His saying: , meaning do not follow it and do not act upon it. It is known that the Muslims, from the time of the Companions, have continued to stop at the reports of single narrators, act upon them, and establish by them matters of the unseen and creedal realities, such as the beginning of creation and the signs of the Hour, and establish by them the attributes of Allah the Exalted. If they did not yield knowledge or establish creed, then the Companions, the Successors, those who followed them, and all the Imams of Islam would have stopped at what they had no knowledge of, as Ibn al-Qayyim (may Allah have mercy on him) said in his book "Mukhtasar al-Sawa'i'q".
Third: His saying: , and in the other reading: . This indicates that if a just person brings a report, the proof is established and it is not obligatory to verify it. Therefore, Ibn al-Qayyim said: "This indicates the certainty of accepting the report of a single narrator, that it does not require verification. If his report did not yield knowledge, [Allah] would have commanded verification until that [knowledge] was obtained." Also indicating this is that the righteous Salaf and the Imams of Islam have continued to say, "The Messenger of Allah (peace and blessings be upon him) said such and such, did such and such, commanded such and such, and forbade such and such." This is known from their speech by necessity. In Sahih al-Bukhari, it is said, "The Messenger of Allah (peace and blessings be upon him) said..." in several places, and many of the hadiths of the Companions contain one of them saying: "The Messenger of Allah (peace and blessings be upon him) said", although he heard it from another Companion. This is a testimony by the speaker... regarding the Messenger of Allah (peace and blessings be upon him) with what is attributed to him of speech or action. If the report of a single narrator did not yield knowledge, then [he would be] a witness to the Messenger of Allah (peace and blessings be upon him) without knowledge. End of quote.
The practical Sunnah followed by the Prophet (peace and blessings be upon him) and his Companions, and after his death, also indicates decisively that there is no distinction between the report of a single narrator in creed and rulings, and that it is an established proof in all of that. I am now mentioning, with Allah's permission, some of the authentic hadiths I have come across regarding this. Imam al-Bukhari said in his Sahih, "Chapter on what has come regarding the acceptance of the report of a truthful single narrator in the call to prayer, prayer, fasting, inheritance, and rulings." And the saying of Allah the Exalted: . A man is called a "group" in His saying: . So if two men fought, they would fall under the meaning of the verse. And His saying: . And how the Prophet (peace and blessings be upon him) sent his governors one after another, and if any of them deviated, he was returned to the Sunnah.