Ibn Katheer said in al-Bidaayah wan-Nihaayah (7/178):

And al-Ash'aree was a Mu'tazilee but he repented from that [whilst] on top of the minbar (pulpit) in Basrah and then he publicized the scandals of the Mu'tazilah.

His books testify to that. He then remained, for a period, speaking in certain matters of belief with the saying of Ibn Kullaab. However, in the final stage of his life he was upon the aqeedah of the Salaf and he spoke with what Imaam Ahmad spoke with - may Allaah have mercy upon him - with respect to all the attributes of Allaah. So he affirmed for Allaah whatever Allaah affirmed for himself and whatever His Messenger (sallallaahu alaihi wasallam) affirmed for Him in his Sunnah. Due to his previous influence and due to his knowing the creed of the Salaf in only a general sense and the ways of the Mu'tazilah in a detailed sense, there were some areas in which he was not able to reach the complete truth.

But his recantation from his past and his adoption of the creed of the Salaf has been recorded in his book called Al-Ibaanah an Usool id-Diyaanah. However, those who ascribed themselves to the madhhab of Abu Hasan al-Ash'aree continued taking from the madhhab of Ibn Kullaab and passed it on to those who ascribed themselves to Abu Hasan al-Ash'aree, generation after generation. The truth, however, is what the scholars testify to, those in whose honesty, integrity and impartiality there is no doubt, that the aqeedah which Abu Hasan al-Ash'aree held and worshipped Allaah with, is the aqeedah which he has affirmed in his book Al-Ibaanah and that this was the last of his books in which the matter of his aqeedah was settled.