Musannaf Ibn Abi Shayba 3:1

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from ‘Abdullah ibn Muhammad ibn ‘Aqil, from 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 (the state of ihram) is the takbir (saying Allahu Akbar), and its release (from the state of prayer) is the taslim (saying as-salamu alaykum)."