Riyad as-Salihin, Book 16, Hadith 1415 (In-Book) ⓘ

Thauban (May Allah be pleased with him) reported: Whenever the Messenger of Allah concluded his prayer, he would beg forgiveness from Allah thrice and then would recite: "Allahumma Antas- Salamu, wa minkas-salamu, tabarakta ya Dhal-Jalali wal-Ikram (O Allah, You are the Grantor of security, and security comes from You. You are Blessing, O You Who have majesty and nobility)!" (Imam) Al-Awza'i, one of the narrators of this Hadith, was asked: "How forgiveness is to be sought?" He answered: "The Messenger of Allah used to say: 'Astaghfirullah! Astaghfirullah! (I beseech Allah for forgiveness, I beseech Allah for forgiveness)'."[Muslim].

وعن ثوبان رضي الله عنه قال‏:‏ كان رسول الله صلى الله عليه وسلم‏:‏ إذا انصرف من صلاته استغفر ثلاثا ، وقال ‏:‏ ‏ "‏ اللهم أنت السلام ومنك السلام تباركت يا ذا الجلال والإكرام “ قيل للأوزاعي ، وهو أحد رواة الحديث ‏:‏ كيف الاستغفار ‏؟‏ قال ‏:‏ يقول ‏:‏ أستغفر الله أستغفر الله ‏" ‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏