Musnad Ahmad 1153:56

Muhammad ibn Abi ‘Adi narrated to us, from Dawud, and Rab‘i ibn Ibrahim, they said: Dawud narrated to us, from al-Sha‘bi, from Masruq, who said: ‘Aishah said: The Messenger of Allah (peace and blessings of Allah be upon him) used to say frequently at the end of his life: "Glory be to Allah and by His praise, I seek forgiveness from Allah, and I repent to Him." She said: So I said, "O Messenger of Allah, why do I see you saying frequently: 'Glory be to Allah and by His praise, I seek forgiveness from Allah, and I repent to Him'?" He said: "Indeed, my Lord, Mighty and Majestic is He, had informed me that I would see a sign in my nation, and He commanded me that when I saw it, I should glorify Him with His praise and seek His forgiveness, for He is the Accepter of Repentance. And I have indeed seen it: 'When the victory of Allah comes and the conquest, and you see the people entering into the religion of Allah in multitudes, then glorify the praise of your Lord and ask His forgiveness. Indeed, He is ever Accepting of repentance.'"