Musnad Abu Ya'la al-Mawsili 42:152

Narrated to us Ishaq, narrated to us Hassan bin Ibrahim, narrated to us Abu Sufyan, from Abu Nadra, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The key to prayer is ablution (wudu), its prohibition (at-tahrim) is the takbir (saying Allahu Akbar), and its release (at-tahlil) is the taslim (saying as-salamu alaykum)."