Musannaf Ibn Abi Shayba 3:5

Abu Bakr narrated to us, he said: Yazid ibn Harun narrated to us, from Husayn al-Mu'allim, from Budayl, from Abu al-Jawza', from 'Aisha, she said: The Prophet, may Allah bless him and grant him peace, used to begin the prayer with the takbir (saying "Allahu Akbar"). And he used to conclude it with the taslim (saying "Assalamu alaikum").