When a man has intercourse with his wife through the vagina but being on her back. the child will have squint, so the verse came down:" Your wives are your tilth; go then unto your tilth as you may desire" (ii. 223) (Using translation from Muslim 1435a)
كَانَتِ الْيَهُودُ تَقُولُ فِي الرَّجُلِ يَأْتِي امْرَأَتَهُ مِنْ قِبَلِ دُبُرِهَا فِي قُبُلِهَا إِنَّ الْوَلَدَ يَكُونُ أَحْوَلَ فَنَزَلَتْ {نِسَاؤُكُمْ حَرْثٌ لَكُمْ فَأْتُوا حَرْثَكُمْ أَنَّى شِئْتُمْ} [البقرة 223]