Musannaf Ibn Abi Shayba 3:2008

Mu'tamir narrated to us, from his father, who said: It reached me from Abu Mijlaz that the Prophet, peace and blessings of Allah be upon him, recited in the Dhuhr prayer and then prostrated, so they perceived that he had recited "Alif Lam Mim Tanzil al-Sajdah."