From Sahih Muslim (Hadith 1088):
A'isha reported:The Messenger of Allah (ﷺ) recited often these words: Hallowed be Allah and with His praise, I seek the forgiveness of Allah and return to Him. She said: I asked: Messenger of Allah, I see that you often repeat the saying" subhan allahi bihamdihi astag firullahi watubuilaih" whereupon he said: My Lord informed me that I would soon see a sign in my Ummah, so when I see it I often recite (these) words: Hallowed be Allah and with His Praise, I seek forgiveness of Allah and return to Him. Indeed I saw it (when this verse) was revealed:" When Allah's help and victory came, it marked the victory of Mecca, and you see people entering into Allah's religion in troops, celebrate the praise of Thy Lord and ask His forgiveness. Surely He is ever returning to Mercy
Reference: Muslim 1088