Musannaf Ibn Abi Shaybah

Hazrat Mujahid says that I traveled from Makkah to Madinah with Hazrat Ibn Umar (may Allah be pleased with them). He used to perform nafl (voluntary) prayers on his mount, regardless of which direction it was facing. However, when he had to perform a fard (obligatory) prayer, he would dismount and perform it on the ground.