Musnad Abu Ya'la al-Mawsili 155:257

Al-Abbas ibn al-Walid al-Narsi narrated to us, Jarir narrated to us, from Umarah ibn al-Qa'qa', from Abu Zur'ah ibn 'Amr ibn Jarir, from Abu Hurayrah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would say the takbir for prayer, he would remain silent for a short while before reciting. I said to him, "May my father and mother be sacrificed for you, O Messenger of Allah, what do you say during this silence of yours between the takbir and the recitation?" He said: "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 snow, water, and hail."