From the devious foundations of the Ash'ari methodology is that it is not permissible to hold on to the lone-reporter narration (al-khabar al-waahid) in knowing Allaah (the Mighty and Majestic).

This foundation is mentioned by the chief of the Ash'ari apologists, Fakhr ud-Din ar-Raazee, and the author of the work, "Asaas ut-Taqdees", a work aimed at refuting the affirmation of Allaah's Attributes.

Ar-Raazee says in Asaas ut-Taqdees (pp.215-219) - this is a lengthy passage, so we will just show the opening part of the chapter:

And we cant translate the opening part of it and a couple of the angles he mentions:

As for holding on to the khabar ul-waahid (lone-reporter narration) in the knowledge of Allaah - the Most High - then it is not permissible, and this is evidenced by numerous (affairs):

The first: That the akhbaar ul-aahaad are presumed (knowledge) only, and thus, it is not permissible to hold on to them in the knowledge of Allaah - the Most High - and His Attributes ...

The second: That the most lofty of the generations of narrators in status and the highest of them in position is [that of] the Companions - may Allaah be pleased with them. We know that their narrations do not amount to definitiveness and certainty ...

And he mentions amongst other reasons to support the notion that the khabar ul-waahid cannot to be taken in affairs relating to the knowledge of Allaah, such as the spread of fabricated narrations and that those Companions who heard from the Messenger (sallallaahu alayhi wasallam) did not write them down but would remember them years later (i.e. casting doubt upon the accuracy of what they narrated).

Shaykh ul-Islaam Ibn Taymiyyah comments on this action of ar-Raazee (in Majmoo al-Fataawaa 1/459):

And it is strange that this man, an opposer to Allaah and His Messenger, has turned towards the narrations, [those] abundantly [transmitted] from the Messenger of Allaah (sallallaahu alayhi wasallam), which have been inherited from him by the Imaams of the religion, and the inheritance of the Prophets and Messengers, and all of the experts are agreed upon their authenticity - and he reviled them in a manner that resembles the revilement of the Heretics (Zanaadiqah) and Hypocrites. Then (after all this) he quotes Abu Ma'shar, one of the believers in al-jibt and at-taaghoot (i.e. magic), [one of the] leaders of shirk and misguidance, as proof in the foundations of the religion. We seek refuge from their evils and their statements, and Allaah is the One whose aid is sought against what they describe (Him) with.

Abu Ma'shar is J'afar bin Muhammad al-Balkhee (d. 272H), who wrote books on astrology and geometry.

This is the way of the deniers of the Attributes - they cast aspersions upon the narrations, either upon their authenticity or upon their contents, and they revile and rebuke much of what has been reported in the Sunnah regarding Allaah, the Most High, and in that, they rely upon the sayings and the principles of the Hellenistic (Greek) Philosophers, the Astrologers, the Materialists and other than them - which is in truth, the fountain from which they have drunk.

Refer to: Bayaan Talbees ul-Jahmiyyah, Qism ud-Diraasah (first book), p.261-263.