Sahih Bukhari Volume 004, Book 055, Hadith Number 606.
Narated By Malik bin Sasaa : Allah's Apostle talked to his companions about his Night Journey to the Heavens. When he reached the fifth Heaven, he met Aaron. (Gabriel said to the Prophet), "This is Aaron." The Prophet said, "Gabriel greeted and so did I, and he returned the greeting saying, 'Welcome, O Pious Brother and Pious Prophet."