Riyad us Saliheen: Hadith 1414

Whenever the Messenger of Allah (ﷺ) 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 (ﷺ) used to say: 'Astaghfirullah! Astaghfirullah! (I beseech Allah for forgiveness, I beseech Allah for forgiveness)'."

[Muslim].