Mentioning some of the evidence from the Qur'an regarding the obligation to accept the report of a single narrator, specifically in matters of creed.

The Shaykh : ... and explaining how it indicates the obligation to accept the report of a single narrator in matters of creed:
First: His saying, the Exalted: . Allah, Blessed and Exalted is He, has warned the believers that a group from among them should not all go to the Prophet (peace and blessings be upon him) to learn their religion from him and gain deep understanding of it. There is no doubt that this is not limited to what is called branches and rulings, but it is broader. Indeed, it is certain that the teacher and the student begin with what is most important, then what is next in importance, in both teaching and learning.
It is beyond 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 noble verse. For just as Allah encouraged the group in it to learn and gain deep understanding of 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 ruling, Allah would not have encouraged the group to convey the message, explaining this by His saying, the Exalted: , which is explicit that knowledge is obtained by the warning of the group. It is like His saying, the Exalted, in His legislative and cosmic verses: , "perhaps they will understand," . So the verse is explicit that the report of a single narrator is a proof in conveying creed and rulings.
Second: His saying, the Exalted: , 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 and act upon them, establishing thereby matters of the unseen and creedal realities; such as the beginning of creation and the signs of the Hour. Indeed, they establish thereby the attributes for Allah, the Exalted. If these reports 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'iq."
Third: His saying, the Exalted: , and in the other reading: . This indicates that if a just person brings a report, the proof is established and it is not necessary 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, He would have ordered verification until that was achieved. Also, what indicates 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, ordered such and such, and forbade such and such. This is known in 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 have one of them saying: The Messenger of Allah (peace and blessings be upon him) said... although he only heard it from another Companion. This is a testimony by the speaker and a certainty regarding the Messenger of Allah (peace and blessings be upon him) regarding what is attributed to him of speech or action. If the report of a single narrator did not yield knowledge, he would be a witness against 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 passing, 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 came 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" (ta'ifah) by His saying, the Exalted: . So if two men fought, they would fall under the meaning of the verse. And His saying, the Exalted: . And how the Prophet (peace and blessings be upon him) sent his commanders one after another, so if one of them stopped, he would be returned to the Sunnah.