Musnad Abi Ya'la 6097 — when the Messenger of Allah ﷺ said the opening

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) said the opening takbir for the prayer, he would remain silent for a short while before beginning the recitation. I said: "O Messenger of Allah (peace and blessings be upon him)! May my father and mother be ransomed for you, what do you recite in this silence of yours between the takbir and the recitation?" He (peace and blessings be upon him) said that he recites this supplication: 'Allahumma ba'id bayni wa bayna khatayaya kama ba'adta baynal-mashriqi wal-maghrib; Allahumma naqqini min khatayaya kama yunaqqath-thawbul-abyadu minad-danas; Allahummaghsilni min khatayaya bith-thalji wal-ma'i wal-barad' (O Allah, put a distance between me and my sins as You have put a distance between the east and the west; O Allah, cleanse me of my sins as a white garment is cleansed of dirt; O Allah, wash away my sins with snow, water and hail).