The Meaning of Voluntary Prayer in the Prophet's Prayer on His Mount

Question:
The questioner Abu Haytham says: It is reported from the Messenger (peace and blessings be upon him) that he used to pray the voluntary prayer on his mount. What is meant by the voluntary prayer here? And what is the mount in our time? Is it the car?

Answer:
The voluntary prayer referred to here is the night prayer. The Prophet (peace and blessings be upon him) used to pray the tahajjud on his mount, wherever his mount took him. This is a favor from Allah upon the Muslims, allowing them to increase their acts of worship while traveling and while resident, so that they do not cease from worship and its reward.
The voluntary prayer referred to is the night prayer. The car is like the mount. The one riding it prays if he is driving it, praying wherever it takes him. As for if he is a passenger, he can face the qiblah; he can turn around and face the qiblah while in the car. And if he cannot turn around, he prays wherever the car takes him.