Musnad Ahmad 5:442

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 (peace and blessings of Allah be upon him) said: "The key to prayer is purification, its prohibition is the Takbir (saying 'Allahu Akbar'), and its release is the Taslim (saying 'Assalamu Alaikum')."