In the book Kashful-Ghummah, as narrated by Faidh al-Kashani, from Umar that when the Prophet (P.B.U.H&H.P) was asked as to what voice did Allah (S.W.T) speak to him during the Journey (Ascension), the Prophet replied:
My Lord spoke to me with the voice of Ali Ibn Abi Talib and said, O Ahmad! I am an Entity that is not like anything else. I cannot be compared to anything else and I know all the secrets of your heart. With the exception of Ali Ibn Abu Talib, you have no other close friend. Thus, I speak to you with the voice of Ali Ibn Abi Talib so that your heart will be at ease.
Ref: duas.org
Ascension - Islam Guidance
