Musnad Ahmad 1153:2383

Asbat ibn Muhammad narrated to us, he said: Shu'bah narrated to us, from Budayl, from Abu al-Jawza', from 'Aishah, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to begin the prayer with the takbir (saying Allahu Akbar), and he used to begin the recitation with 'Al-hamdu lillahi' (Praise be to Allah)."