Whenever the Messenger of Allah (ﷺ) concluded his prayer, he would beg forgiveness from Allah thrice

Riyad as-Salihin The Book of the Remembrance of Allah Sunnah.com reference : Book 16, Hadith 8

Thauban (May Allah be pleased with him) reported: 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].