What is the ruling on the prayer of someone who heard the Maghrib call to prayer while in a remote desert area, with a mosque one kilometer away and his residence 50 kilometers away, and the father and his son prayed, while the father's companion did not cleanse himself from urine? Should he pray with tayammum or not, and will he miss the prayer?

Tayammum (definition) is prescribed when water is unavailable or when there is a fear of harm from using it, based on the Almighty's saying: "And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and hands [with it]." So, whoever finds water is obliged to perform ablution. If one does not find water but is able to obtain it before the prayer time expires, then he should delay the prayer for that reason, as "praying at the end of the time for one hoping for water is preferable." However, if one fears the prayer time will expire before obtaining water, then he should perform tayammum.

As for istinja (definition) (cleansing after defecating or urinating), a person has a choice after relieving himself between cleansing with water or performing istijmar with stones, as long as the excrement has not exceeded its point of exit.