Sunan Ibn Majah
928
Sahih+3

Thawban narrated that when he finished his prayer, the Messenger of Allah (ﷺ) would ask for forgiveness three times, then he would say:“Allahumma Antas-Salam wa minkas-salam tabarakta ya Dhal-jalali wal- ikram” (O Allah, You are As-Salam and from You is all peace, Blessed are You O Possessor of majesty and honour).”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi