Musannaf Ibn Abi Shayba 3:2455

Narrated to us Sharik, from Jabir, from al-Hakam, from Ali, who said: If he intends to say "Sami' Allahu liman hamidah" (Allah hears those who praise Him), but instead says "Allahu Akbar" (Allah is the Greatest), he should seek forgiveness from Allah.