Prayer: Probability of Completing the Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 868
Prayers performed on a pile of wheat, barley, and similar things on which one cannot remain still are invalid.
The third condition: one must perform prayers in a place where he deems it probable that he will complete them. However, if one is confident that he will not be able to complete his prayers in a place on account of wind, rain, or there being a lot of people around and suchlike, he must perform prayers with the intention of rajāʾ; and if he happens to complete his prayer, it will be valid.