Sahih Bukhari Volume 1, Book 11, Hadith Number 669

Narated 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).