Musnad Abu Ya'la al-Mawsili 3:356

Zuhayr narrated to us, Waki' narrated to us, Sufyan narrated to us, from 'Abdullah ibn Muhammad ibn 'Aqil, from Muhammad ibn al-Hanafiyyah, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The key to prayer is purification, its prohibition is the Takbir (saying 'Allahu Akbar'), and its release is the Taslim (saying 'Assalamu 'alaykum')."