Musnad Ahmad 31:710

Waki’ narrated to us, Ibn Abi Layla narrated to us, from ‘Ata’ or ‘Atiyyah, from Abu Sa’id, and from Nafi’, from Ibn ‘Umar: "That the Prophet, peace and blessings of Allah be upon him, used to pray his voluntary prayers upon his riding beast, wherever it faced him. He would gesture with a gesture, and he would make the prostration lower than the bowing." ‘Abdullah said: "And the correct one is ‘Atiyyah."