Musannaf Ibn Abi Shayba 3:4

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Ibn Kurayb, from his father, from Ibn Abbas, who said: The key to prayer is purification, its prohibition is the Takbir (saying Allahu Akbar), and its release is the Taslim (saying As-salamu alaykum).