Anas bin Malik narrated that fifty prayers, were made obligatory on the Prophet on the night of mi'raj. Then they were reduced till five remained. Then, a voice called, "O Muhammad our word is never changed. For you, indeed, with these five will accrue (reward of) fifty."[Sunan Tirmidhi (1/417) No. 213, Musnad Ahmad (3/161) No. 12662, al-Bukhari (3207) and Muslim (162)]