بُلُوغُ الْمَرَامِ
Bulugh al-Maram 323
The Book of Prayer
وَعَنْ ثَوْبَانَ - رضى الله عنه - قَالَ : { كَانَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -إِذَا اِنْصَرَفَ مِنْ صَلَاتِهِ اِسْتَغْفَرَ اَللَّهَ ثَلَاثًا , وَقَالَ : " اَللَّهُمَّ أَنْتَ اَلسَّلَامُ وَمِنْكَ اَلسَّلَامُ . تَبَارَكْتَ يَا ذَا اَلْجَلَالِ وَالْإِكْرَامِ } رَوَاهُ مُسْلِمٌ . [1]
Narrated Thauban (RA): Whenever Allah's Messenger (ﷺ) finished his prayer, he used to say after asking Allah's forgiveness three times: "O Allah, You are as-Salam (One Free from every defect), and as-Salam (safety from every evil) is (sought) from You. Blessed are You, O Possessor of glory and honour." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 216