What should a ritually impure person (junub) do if they can only find enough water for ablution (wudu')?

If a Muslim becomes impure (junub) and does not have enough water for a full ritual bath (ghusl), he should use the water he has, then perform tayammum (dry ablution) for the remaining parts of his body. This is the view of the Hanbalis and Shafi'is. Their evidence for this is that the Qur'an and Sunnah stipulated tayammum when water is unavailable, and in this case, some water is present, so it must be used first. They also cite the Prophet's (peace be upon him) saying: "When I command you to do something, do as much of it as you can."

Conversely, the Hanafis and Malikis hold that he should only perform tayammum and leave the small amount of water, because it will not purify him completely.

It should be noted that the obligation in ghusl is to apply water to the entire body, and this usually requires only a small amount of water for someone who is skilled in its use.