Abū Sa'īd b. al-Mu‘allā said

When I was praying in the mosque the Prophet (ﷺ) called me and I did not answer him, but I went to him afterwards and explained that I had been praying, whereupon he asked me whether Allah had not said, “Respond to Allah and to the messenger when He calls you,” (Qur’ān, 8:24) adding, “Let me teach you the greatest sūra in the Qur’ān before you leave the mosque.” He then took me by the hand, and when we were about to go out I reminded him of saying he would teach me the greatest sūra in the Qur’ān. He said, “It is, ‘Praise be to Allah, the Lord of the universe’, (Qur'ān,1) which is the seven oft-repeated verses and the mighty Qur’ān’ (Qur’ān, 15:87) which has been brought to me.” Bukhārī transmitted it.