Musnad Ahmad 5:508

'Abd al-Rahman narrated to us, from Sufyan, from 'Abd Allah ibn Muhammad ibn 'Aqil, from Muhammad ibn al-Hanafiyyah, from 'Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The key to prayer is ablution (wudu), its prohibition (the state of sanctity) is the takbir (saying 'Allahu Akbar'), and its release (from the state of sanctity) is the taslim (saying 'as-salamu 'alaykum')."