Sahih Bukhari Volume 1, Book 10, Hadith 701

قَالَ وَحَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا غُنْدَرٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرٍو، قَالَ سَمِعْتُ جَابِرَ بْنَ عَبْدِ اللَّهِ، قَالَ كَانَ مُعَاذُ بْنُ جَبَلٍ يُصَلِّي مَعَ النَّبِيِّ صلى الله عليه وسلم ثُمَّ يَرْجِعُ فَيَؤُمُّ قَوْمَهُ، فَصَلَّى الْعِشَاءَ فَقَرَأَ بِالْبَقَرَةِ، فَانْصَرَفَ الرَّجُلُ، فَكَأَنَّ مُعَاذًا تَنَاوَلَ مِنْهُ، فَبَلَغَ النَّبِيَّ صلى الله عليه وسلم فَقَالَ ‏"‏ فَتَّانٌ فَتَّانٌ فَتَّانٌ ‏"‏ ثَلاَثَ مِرَارٍ أَوْ قَالَ ‏"‏ فَاتِنًا فَاتِنًا فَاتِنٌ ‏"‏ وَأَمَرَهُ بِسُورَتَيْنِ مِنْ أَوْسَطِ الْمُفَصَّلِ‏.‏ قَالَ عَمْرٌو لاَ أَحْفَظُهُمَا‏

Narrated By 'Amr : Jabir bin 'Abdullah said, "Mu'adh bin Jabal used to pray with the Prophet and then go to lead his people in prayer Once he led the 'Isha prayer and recited Surat "Al-Baqra." Somebody left the prayer and Mu'adh criticized him. The news reached the Prophet and he said to Mu'adh, 'You are putting the people to trial,' and repeated it thrice (or said something similar) and ordered him to recite two medium Suras of Mufassal." ('Amr said that he had forgotten the names of those Suras).