The Jews used to say that if a man had intercourse with his wife from behind, the child would be born cross-eyed. So the verse was revealed, "Your wives are a tilth for you, so come to your tilth as you wish" [Baqarah 2:223].
حدثنا أبو نعيم حدثنا سفيان عن ابن المنكدر سمعت
كانت اليهود تقول إذا جامعها من ورائها جاء الولد أحول فنزلت {نساؤكم حرث لكم فأتوا حرثكم أنى شئتم}