Musannaf Ibn Abi Shayba 21:88

Ibn Fudayl narrated to us, from 'Umarah ibn al-Qa'qa', from Abu Zur'ah, from Abu Hurayrah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would say the takbir, he would remain silent between the takbir and the recitation. He said: So I said to him, "May my father and mother be sacrificed for you, what do you say during your silence between the takbir and the recitation? Inform me." He said: "I say: O Allah, distance me from my sins as You have distanced the East from the West. O Allah, purify me from my sins as a white garment is purified from filth. O Allah, wash away my sins with water, hail, and snow."