Thawban, Allah be pleased with him narrates that when the Messenger of Allah would complete his Salah he would seek Allah’s forgiveness three times and say:

O Allah, You are free from all defects and the grantor of security, Blessed are You, O owner of greatness and majesty.

It was asked from Al-Awzai (one of the narrators of the Hadith), How is Allah’s forgiveness to be sought (with which words). He replied that the individual should say ‘Astaghfirullah,Astaghfirullah’.

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،ح/١٤١٥)