Musnad Ahmad 32:698

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, he said: Anas ibn Malik informed me, he said: "The prayers were made obligatory upon the Prophet, may Allah bless him and grant him peace, on the night he was taken on the Night Journey, as fifty. Then they were reduced until they were made five. Then it was called out: 'O Muhammad, indeed the word is not changed with Me, and indeed for you, with these five, is [the reward of] fifty.'"