Musannaf Ibn Abi Shayba 3:3

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Abu Sufyan al-Sa'di, from Abu Nadra, from Abu Sa'id al-Khudri, he said: The Prophet, may Allah bless him and grant him peace, said: "The key to prayer is purification, its prohibition is the takbir (saying 'Allahu Akbar'), and its release is the taslim (saying 'Assalamu alaikum')."