Riyad as-Salihin, Book 20, Hadith 1876 (In-Book) ⓘ

Thauban (May Allah be pleased with him) reported: Whenever the Messenger of Allah finished his Salat (prayer), he would beg forgiveness three times [by saying, 'Astaghfirullah' (3 times)] and then he would say: "Allahumma Antas-Salamu, wa minkas-Salamu, tabarakta ya Dhal-Jalali wal-Ikram. (O Allah! You are the Bestower of security and security comes from You; Blessed are You. O Possessor of glory and honour)." Imam Al-Auza'i (one of the subnarrators) of this Hadith was asked: "How forgiveness should be sought?" He replied: "I say: Astaghfirullah, Astaghfirullah (I seek forgiveness from Allah. I seek forgiveness from Allah)."[Muslim].

- وعن ثوبان رضي الله عنه قال‏:‏ كان رسول الله صلى الله عليه وسلم إذا انصرف من صلاته ، استغفر الله ثلاثاً وقال‏:‏ ‏"‏اللهم أنت السلام، ومنك السلام، تباركت يا ياذا الجلال والإكرام‏"‏ قيل للأوزاعي- وهو أحد رواته‏:‏ كيف الإستغفار‏؟‏ قال‏:‏ يقول‏:‏ ‏"‏أستغفر الله ، أستغفر الله‏"‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏