Musnad Ahmad 939:18

Surayj narrated to us, Abd al-Aziz—meaning Ibn al-Darawardi—narrated to us, from Zayd ibn Aslam, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs two prostrations (i.e., a prayer) in which he does not become forgetful, Allah will forgive him his past sins."