Told about a sūrah before leaving the mosque
He took him by the hand and nearly walked out before saying it.
{الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ} هِيَ السَّبْعُ الْمَثَانِي وَالْقُرْآنُ الْعَظِيمُ الَّذِي أُوتِيتُهُ
I will teach you the greatest sūrah in the Qur'an before you leave the mosque. Then as he was about to go out, he said: it is all praise is for Allah, Lord of the worlds, the seven oft-repeated and the great Qur'an that I was given.
The full narration
Narrated Abu Sa`id bin Al-Mu'alla:While I was praying in the Mosque, Allah's Messenger (ﷺ) called me but I did not respond to him. Later I said, "O Allah's Messenger (ﷺ)! I was praying." He said, "Didn't Allah say'--"Give your response to Allah (by obeying Him) and to His Apostle when he calls you." (8.24) He then said to me, "I will teach you a Sura which is the greatest Sura in the Qur'an, before you leave the Mosque." Then he got hold of my hand, and when he intended to leave (the Mosque), I said to him, "Didn't you say to me, 'I will teach you a Sura which is the greatest Sura in the Qur'an?' He said, "Al-Hamdu-Li l-lah Rabbi-l-`alamin (i.e. Praise be to Allah, the Lord of the worlds) which is Al-Sab'a Al-Mathani (i.e. seven repeatedly recited Verses) and the Grand Qur'an which has been given to me
Source
Ṣaḥīḥ al-Bukhārī · 4474
- Agreed upon as authentic (al-Bukhārī / Muslim)