Due to the difference between obligatory and voluntary prayers, if at that time he is not engaged in a confrontation, fierce battle with the enemy, or attacking the enemy (because in these situations, it is permissible to perform even the obligatory prayer on a mount).
Sayyiduna Jabir (may Allah be pleased with him) narrates that we were with the Noble Prophet (peace and blessings be upon him) in a battle. He (peace be upon him) would perform voluntary prayers on his mount facing towards the east. Then, when he wished to perform the obligatory prayer, he would dismount and pray facing the Qiblah. Imam Abu Bakr (may Allah have mercy on him) says that Muhammad bin Thawban refers to Muhammad bin Abdur Rahman bin Thawban; his attribution is to his grandfather Thawban instead of his father.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الفريضة فى السفر / 976