Authentic
Sunan Abu Dawud 1458
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…
أَلَمْ يَقُلِ اللَّهُ عَزَّ وَجَلَّ { يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ } لأُعَلِّمَنَّكَ أَعْظَمَ سُورَةٍ مِنَ الْقُرْآنِ أَوْ فِي الْقُرْآنِ "…