Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.021A.

This prevents dirt from getting on one’s penis, and also makes it easy to keep it clean. For women, it involves cutting the outer portion of the clitoris. Abu Hurairah reported that the Messenger of Allah said, “Ibrahim circumcised himself after he was eighty years old.” (Related by al-Bukhari.) Many scholars say that it is obligatory.’ The Shaf’iyyah maintain that it should be done on the seventh day. Says ash-Shaukani, “There is nothing that states explicitly its time or indicates that it is obligatory.”