The fourth condition: The ceiling of the place where one prays should not be so low, that one may not be able to stand erect, nor should the place be so small, that there may be no room for performing Ruku’ or Sajdah.

894. If a person is forced to offer prayers at a place where it is not at all possible to stand, he will pray while sitting. And if it is not possible to perform Ruku’ and Sajdah, he should perform them by head signs.

895. One should not offer prayers in front of the graves of the holy Prophet, and the holy Imams, if it entails irreverence, otherwise there is no harm in it.