Asked at the very start of the prayer
Abū Hurayrah noticed him ﷺ pausing before reciting, and asked what he was saying.
أَقُولُ اللَّهُمَّ بَاعِدْ بَيْنِي وَبَيْنَ خَطَايَاىَ كَمَا بَاعَدْتَ بَيْنَ الْمَشْرِقِ وَالْمَغْرِبِ، اللَّهُمَّ نَقِّنِي مِنَ الْخَطَايَا كَمَا يُنَقَّى الثَّوْبُ الأَبْيَضُ مِنَ الدَّنَسِ، اللَّهُمَّ اغْسِلْ خَطَايَاىَ بِالْمَاءِ وَالثَّلْجِ وَالْبَرَدِ
Allāhumma bāʿid baynī wa bayna khaṭāyāya kamā bāʿadta bayna-l-mashriqi wa-l-maghrib. Allāhumma naqqinī min khaṭāyāya kamā yunaqqa-th-thawbu-l-abyaḍu mina-d-danas.
“O Allah, put distance between me and my sins as You have put distance between the east and the west. O Allah, cleanse me of my sins as a white garment is cleansed of dirt.”
The full narration
Narrated Abu Huraira:Allah's Messenger (ﷺ) used to keep silent between the Takbir and the recitation of Qur'an and that interval of silence used to be a short one. I said to the Prophet (ﷺ) "May my parents be sacrificed for you! What do you say in the pause between Takbir and recitation?" The Prophet (ﷺ) said, "I say, 'Allahumma, baa`id baini wa baina khatayaya kama baa`adta baina l-mashriqi wa l-maghrib. Allahumma, naqqini min khatayaya kama yunaqqa th-thawbu l-abyadu mina d-danas. Allahumma, ighsil khatayaya bi l-maa'i wa th-thalji wa l-barad (O Allah! Set me apart from my sins (faults) as the East and West are set apart from each other and clean me from sins as a white garment is cleaned of dirt (after thorough washing). O Allah! Wash off my sins with water, snow and hail
Source
Ṣaḥīḥ al-Bukhārī · 744
- Agreed upon as authentic (al-Bukhārī / Muslim)