Musannaf Ibn Abi Shayba 9:762

Marwan ibn Mu'awiyah narrated to us, from Isma'il, from Abu Salih, regarding [the verse]: "Your women are a tilth for you, so come to your tilth when or how you will." He said: If you wish, come to her while she is lying on her back, and if you wish, while she is moving, and if you wish, while she is kneeling.