Thauban (May Allah be pleased with him) reported:
Whenever the Messenger of Allah ﷺ 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 ﷺ 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)).