Sahih Muslim Book - 5, Hadith - 1334

حَدَّثَنَا دَاوُدُ بْنُ رُشَيْدٍ حَدَّثَنَا الْوَلِيدُ عَنِ الأَوْزَاعِىِّ عَنْ أَبِى عَمَّارٍ - اسْمُهُ شَدَّادُ بْنُ عَبْدِ اللَّهِ - عَنْ أَبِى أَسْمَاءَ عَنْ ثَوْبَانَ قَالَ كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- إِذَا انْصَرَفَ مِنْ صَلاَتِهِ اسْتَغْفَرَ ثَلاَثًا وَقَالَ « اللَّهُمَّ أَنْتَ السَّلاَمُ وَمِنْكَ السَّلاَمُ تَبَارَكْتَ ذَا الْجَلاَلِ وَالإِكْرَامِ ». قَالَ الْوَلِيدُ فَقُلْتُ لِلأَوْزَاعِىِّ كَيْفَ الاِسْتِغْفَارُ قَالَ تَقُولُ أَسْتَغْفِرُ اللَّهَ أَسْتَغْفِرُ اللَّهَ.

It was narrated that Thawban said: "When the Messenger of Allah (s.a.w) had finished his prayer, he would ask for forgiveness three times, and say: 'Allahumma antas-Salamu wa minkas-salamu, tabarakta dhal-jalali wal-ikriim (O Allah, You are As-Salam (the One Who is free from all defects and deficiencies) and from You is all peace, blessed are You, Possessor of majesty and honor)." Al-Walid said: "I said to Al-Awza'i: 'How does one ask for forgiveness?' He said: 'Say: ''Astaghfirullah, astaghfirullah (I ask Allah for forgiveness, I ask Allah for forgiveness.)"