سنن أبي داود

Abu Sa'id b. al-Mu'alla said that when he was praying the Prophet (ﷺ) passed by him and he called him. He said: I prayed ant then I came to him. He asked: What prevented you from answering me ? He replied: I was praying. He said: Has not Allah said: "O you who believe, respond to Allah and the Apostle when he calls you to that which gives you life ? (8:24) Let me teach you the greatest surah from the Qur'an or in the Qur'an (the narrator Khalid doubted) before I leave the mosque. I said: (I shall memorize) your saying. He said: It is: "Praise be to Allah, the Lord of the Universe" which is the seven oft-repeated verses, and the mighty Qur'an.