Sahih Muslim Book 11, Hadith Number 3940.

Chapter : The last verse revealed was that pertaining to Kalala.

Abu Ishaq said that he heard al-Bara’ b. ‘Azib (Allah be pleased with him say: The last verse revealed (in the Holy Quran) is that pertaining to Kalala, and the last sura revealed is Sura al-Bara’at.