Ruling on offering an obligatory prayer on the bus when it is not necessary to do so, when he does all the essential parts of the prayer
Question

                I am a bus driver and sometimes I have to offer an obligatory prayer during my short rest period on my empty bus. I have a large space where I can face towards the qiblah and do all the essential parts (“pillars”) of the prayer. But I was surprised when a brother told me that the bus comes under the same ruling as a mount and it is not permissible to offer an obligatory prayer on it; rather he said I should exit the bus and pray on the street, so long as there is nothing to prevent me from doing so such as rain and so on.

Is it true that I should not pray on the bus, which is thirteen meters long and has space in the middle that is enough for me to pray, when it is parked in the garage?.
Praise be to Allah.It is not 
    permissible for a Muslim to offer an obligatory prayer on his mount, unless 
    there is a valid excuse, because on a mount he will omit some of the 
    essential parts of the prayer, such as standing, bowing and prostrating. 
It was narrated 
    from Jaabir ibn ‘Abdullah (may Allah be pleased with him) that the Prophet 
    (blessings and peace of Allah be upon him) used to pray on his mount facing 
    towards the east, but when he wanted to offer an obligatory prayer, he would 
    dismount and face towards the qiblah. 
Ibn Battaal said: 
    The scholars are unanimously agreed that it is not permissible for anyone to 
    offer the obligatory prayer on his mount without a valid excuse. Because 
    offering obligatory prayers on one’s mount when it is possible to dismount 
    is not permissible, and because the conditions of the obligatory prayer 
    stipulate that the worshipper should face towards the qiblah and continue to 
    do so throughout, an obligatory prayer offered by a rider who cannot stand 
    up or face the qiblah is not valid.
End quote from 
    al-Mawsoo‘ah al-Fiqhiyyah, 27/231 
But this ruling 
    does not apply to one who prays on a bus when he is able to do all the 
    obligatory and essential parts of the prayer, such as facing towards the 
    qiblah, standing, bowing and prostrating. The scholars have stated that 
    praying in a howdah is valid, because one can do all the obligatory parts of 
    the prayer. The howdah is like a room on the camel’s back. 
An-Nawawi (may 
    Allah have mercy on him) said: The conditions of the obligatory prayer are 
    that the worshipper should face towards the qiblah and continue to do so 
    throughout. It is not valid without facing towards the qiblah except in 
    cases of extreme fear, and it is not valid on the part of one who is walking 
    whilst facing towards the qiblah, or one who is riding and is not able to 
    stand up or face towards the qiblah. And there is no difference of scholarly 
    opinion on this point. 
If he faces 
    towards the qiblah and does all the essential parts of the prayer in the 
    howdah or similar place on the back of a mount that is standing, there are 
    two opinions concerning the validity of his obligatory prayer. The more 
    correct opinion is that it is valid and this was stated definitively by the 
    majority.
End quote from 
    Sharh al-Muhadhdhab, 3/221 
It says in 
    al-Mawsoo‘ah al-Fiqhiyyah (27/231): If the obligatory prayer is offered 
    on one’s mount, that is not permissible unless there is an excuse, because 
    the conditions of the obligatory prayer are that the worshipper should be 
    facing towards the qiblah and continue to do so throughout, and he should 
    fulfil all the conditions and essential parts. But if a person is able to 
    offer the obligatory prayer atop his mount, fulfilling all of its conditions 
    and doing all of its essential parts, even if he has no excuse, his prayer 
    is valid. This is the view of the Shaafa‘is and Hanbalis, and it is the 
    preferred view of the Maalikis.
End quote. 
Based on that, 
    there is nothing wrong with you praying on the bus so long as you do all the 
    essential parts of the prayer. 
But it must be 
    noted that it is obligatory to offer the obligatory prayer with the Muslim 
    congregation in the houses of Allah (i.e., in the mosques) and it is not 
    permissible to stay away from offering the prayer in congregation in the 
    mosque except for one who has an excuse. For more information please see the 
    answer to question no. 8918 and
    40299. 
And Allah knows best.