Aishah (RA) said: 'Allah's Messenger (sal-Allaahu 'alayhe wa sallam) did not sit in a gathering, and did not recite the Qur'an, and did not perform any prayer without concluding by saying:
سُبْحَانَكَ اللَّهُمَّ وَبِحَمْدِكَ، أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ أَنْتَ، أَسْتَغْفِرُكَ وَأَتُوبُ إِلَيْكَ
Glory is to You, O Allah, and praise is to You. I bear witness that there is none worthy of worship but You. I seek Your forgiveness and repent to You.
Subhaanaka Allaahumma wa bihamdika, 'ash-hadu 'an laa 'ilaaha 'illaa 'Anta, 'astaghfiruka wa 'atoobu 'ilayka.
Abu Dawud, Ibn Majah, At-Tirmithi and An-Nasa'i. See also Al-Albani, Sahih At-Tirmithi 3/ 153. This was reported by An-Nasa'i in 'Amalul-Yawm wal-Laylah (no.308), and Dr. Farooq Hamadah graded it authentic in his checking of the same book, p. 273. See also Ahmad 6/77