Musnad Ahmad 1153:1364

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Budayl, from Abu al-Jawza', from 'Aishah, that she said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to begin his prayer with the takbir (saying 'Allahu Akbar'), and he used to begin the recitation with 'Al-hamdu lillahi rabbil-'alamin' (Praise be to Allah, Lord of the worlds), and he used to conclude it with the taslim (saying 'As-salamu 'alaykum')."