Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3365
This hadith has been reported on the authority of Jabir through another chain of transmitters, but in the hadith transmitted on the authority of Zuhri there is an addition (of these words):" If he likes he may (have intercourse) being on the back or in front of her, but it should be through one opening (vagina)."