Reference: Fataawa az-Zawaaj wa ‘Ishratun-Nisaa. – Question 75, Page 106
Question: Is it permissible for a man to enter his wife (have sexual intercourse) in her vagina from behind?
Response: All praise is for Allaah [alone].
It is certainly permissible for the man to penetrate his wife in her vagina from behind, since this is the place of pregnancy, and it is for him to enter her [in her vagina] in any way [he pleases], as Allaah (Subhaanahu wa Ta’aala) says:
{Your wives are a tilth for you, so go to your tilth, when or how you will…}, [Soorah al-Baqarah, Aayah 223]