Musannaf Ibn Abi Shayba 3:2

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Abu Ishaq, from Abu al-Ahwas, who said: Abdullah said: The prohibition of the prayer is the Takbir (saying Allahu Akbar), and its permissibility is the Taslim (saying As-salamu alaykum).