The Anus Is a Place of Severe Filth and Impurity- by Shaikhul Islaam Ibn Taymiyyah
Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) said: Indeed it has being reported from the Prophet (sallal-laahu-alayhi-wasallam) that he said: ‘’Indeed, Allaah is not shy of (stating) the truth, do not approach your women (i.e.sexual intercourse with your wives) in their anuses.’’
The anus is a place of filth (i.e. the exit place of poo or excrement). Allaah (Glorified and Exalted Be He) forbids (a husband) from having sexual intercourse with a menstruating woman (i.e. his wife) due to the impurity in her vagina, then how about having sexual intercourse in the place (i.e. anus) from which (exits) severe impurity.
[Majmoo Al-Fataawaa 32/267]