Musnad Ahmad 1015:19

Abu al-Nadr narrated to us, Abu Mu'awiyah—meaning Shayban—and Layth narrated to us, from Shahr ibn Hawshab, from Abu Malik al-Ash'ari, from the Messenger of Allah (peace and blessings of Allah be upon him): "He used to make the four rak'ahs equal in recitation and standing, and he would make the first rak'ah the longest of them so that the people would gather. He would place the men in front of the boys, the boys behind them, and the women behind the boys. He would say 'Allahu Akbar' whenever he prostrated and whenever he rose, and he would say 'Allahu Akbar' whenever he stood up between the two rak'ahs if he had been sitting."