Musannaf Ibn Abi Shayba 3:6128

Waki’ narrated to us, he said: Ibn Abi Layla narrated to us, from ‘Atiyyah, from Abu Sa’id, and from Ibn Abi Layla, from Nafi’, from Ibn ‘Umar, from the Prophet (may Allah bless him and grant him peace), that he used to pray the voluntary prayer on his riding beast while traveling, in whichever direction it faced him, gesturing with a gesture; [making] the prostration lower than the bowing.