The Whispering Of The Soul Or The Wandering Of One's Thoughts Are Disliked During Prayer Sunan Abi Dawud 905
Translation
Zaid b. Khalid al-Juhani reported the Messenger of Allah (ﷺ) as saying
Whoever performs ablution and performs his ablution well, then prays two rak'ahs without becoming forgetful in them, his past sins will be forgiven.