Sunan An-Nasai, Chapter: The Book of Purification

"When the Messenger of Allah (saw) started Salah, he would remain silent for a short while. I said: 'May my father and mother be ransomed for you, O Messenger of Allah! What do you say when you remain silent between the Takbir and the recitation (in the Salah)?' He said: 'I say: Allahumma ba'id baini wa baina khatayaya kama ba'adta baina al-mashriq wal-maghrib; Allahumma naqqini min khatayaya kama yunaqqath-thawb al-abyad min ad-danas; Allahummaghsilni min khatayaya bith-thalji wal-ma'i wal-barad (O Allah, put a great distance between me and my sins, as great as the distance You have made between the East and the West; O Allah, cleanse me of sin as a white garment is cleansed from filth; Wash away my sins with snow, water, and hail)'." (Sahih)