889. The second condition: The place for prayers should not have such a vigorous movement which would make normal standing, Ruku’ or Sajdah impossible. In fact, as an obligatory precaution, it should not prevent the body from being at ease.
But if one is forced to pray at such places, due to shortage of time, or any other reason, like in a car, on a ship or on train, then one should try to remain still, and to maintain the direction of Qibla, as much as possible. And if the vehicles move away from the direction, he should return to Qibla.

890. There is no harm in offering prayers in a car or a boat, or on railway train or other vehicles, while they are motionless. And if they do not cause excessive swaying to the body, when they are in motion, one can pray in them.

891. Prayers offered on a heap of wheat, or barley, or any other similar thing, which cannot remain steady, is void.