Musnad Abu Ya'la al-Mawsili 42:104

Abu Sa’id said: The Messenger of Allah (ﷺ) said: "The key to prayer is purification (wudu), its prohibition (at-tahrim) is the takbir (saying Allahu Akbar), and its release (at-tahlil) is the taslim (saying as-salamu alaykum). And in every two rak'ahs there is a taslim. And a prayer is not valid in which the Opening of the Book (Al-Fatiha) and something else with it is not recited."