It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever approaches his wife from her back passage, Allah will not look at him with mercy.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7684
Hadith Gradingحکم دارالسلام:حديث حسن لغيره، وهذا إسناد ضعيف، لجهالة الحارث.