Musnad Ahmad 25:1032

Aswad ibn Amir narrated to us, Sharik narrated to us, from Abdullah ibn 'Isam, from Ibn Abbas, who said: "Allah, the Mighty and Majestic, obligated the prayer upon His Prophet, may Allah bless him and grant him peace, as fifty prayers. Then he asked his Lord, the Mighty and Majestic, so He made them five prayers."