Sahih Muslim Book 4, Hadith Number 982.

Chapter : Following the Imam and acting after him.

‘Aisha reported that the Messenger of Allah (may peace be upon him) before his death recited often: Hallowed be Thou, and with Thy praise, I seek forgiveness from Thee and return to Thee. She reported: I said: Messenger of Allah, what are these words that I find you reciting? He said: There has been made a sign for me in my Ummah; when I saw that, I uttered them (these words of glorification for Allah), and the sign is: “When Allah’s help and victory… to the end of the surah.