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

Section : When water is nearby, but one does not want to fetch it due to fear.

If one fears for his life, family, wealth, (for example, if an enemy is nearby–beast or human–or one is a prisoner, and so on), one may perform tayammum. This is also allowed if there is water but one lacks the proper means to get it, or if one fears some accusation against him if he gets it.