Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.058.

Section : Washing all bodily parts.

This is based on the following: Says Allah, “If you are sexually impure, cleanse yourselves,” that is, perform ghusl, and “They ask you concerning menstruation. Say: ‘It is an illness, so leave women alone at such times and go not in unto them until they are cleansed,” that is, until they perform ghusl. The proof that cleansing means ghusl is in the verse, “O you who believe, draw not unto the prayer when you are drunk until you know what you utter, nor when you are polluted, save when journeying upon the road, until you have bathed (taghtasilu).” This shows that ghusl, the washing of all bodily parts, is meant.