Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.065B.

This could be for a hound, for dough, cooking or to remove an impurity that is not pardonable. Says Imam Ahmad, “Many of the companions performed tayammum to save their water for drinking.” ‘Ali said that a man who is travelling and becomes unclean because of sex or a wet dream can perform tayammum if he fears he will go thirsty: “He should perform tayammum and not ghusl.” (Related by ad-Daraqutni.) Says Ibn Taimiyyah, “If a person needs to relieve himself but has only a small amount of water, it is best that he pray with tayammum and relieve himself, rather than keep his ablution and pray before relieving himself.”