Musannaf Ibn Abi Shayba 3:209

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Husayn al-Maktab, from Budayl, from Abu al-Jawza’, from ‘Aisha, she said: When the Prophet, peace and blessings of Allah be upon him, would bow, he would neither raise his head nor lower it; he would keep it between that.