Imam Ahmad recorded that `Abdullah bin Sabit said: I went to Hafsah bint `Abdur-Rahman bin Abu Bakr and said, "I wish to ask you about something, but I am shy.'' She said, "Do not be shy, O my nephew.'' He said, "About having sex from behind with women.'' She said, "Umm Salamah told me that the Ansar used to refrain from having sex from behind (in the vagina). The Jews claimed that those who have sex with their women from behind would have offspring with crossed-eyes. When the Muhajirun came to Al-Madinah, they married Ansar women and had sex with them from behind. One of these women would not obey her husband and said, `You will not do that until I go to Allah's Messenger (and ask him about this matter). ' She went to Umm Salamah and told her the story. Umm Salamah said, `Wait until Allah's Messenger comes.' When Allah's Messenger came, the Ansari woman was shy to ask him about this matter, so she left. Umm Salamah told Allah's Messenger the story and he said: (Summon the Ansari woman.)'' She was summoned and he recited this Ayah to her: ( Your wives are a tilth for you, so go to your tilth, when or how you will.) He added: (Only in one valve (the vagina))''

source:

Ahmad 6:305