Musnad Ahmad 30:3278

Jarir 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 in prayer, he would remain silent for a brief moment. I said to him: "O Messenger of Allah, may my father and mother be sacrificed for you, what do you say during your silence between the Takbir and the recitation?" 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 me from my sins with snow, water, and hail.'"