Hadeeth: “Indeed, Allaah is not shy of [stating] the Truth” – 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 excrement]. Allaah [Glorified be He and free is He from all imperfections] 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]