Musnad Ahmad 1156:129

'Affan narrated to us, Wuhayb narrated to us, 'Abdullah ibn 'Uthman ibn Khuthaym narrated to us, from 'Abd al-Rahman ibn Sabit, who said: I entered upon Hafsah bint 'Abd al-Rahman and said: "I am going to ask you about a matter, and I am shy to ask you about it." She said: "Do not be shy, O son of my brother." He said: "About approaching women in their posteriors?" She said: "Umm Salamah narrated to me that the Ansar used to not approach women [in that manner], and the Jews used to say: 'Indeed, whoever approaches his wife [in that manner], his child will be cross-eyed.' When the Muhajirun came to Medina, they married women of the Ansar and approached them [in that manner]. A woman refused to obey her husband and said to her husband: 'You shall not do that until I go to the Messenger of Allah, peace and blessings of Allah be upon him.' She entered upon Umm Salamah and mentioned that to her. She said: 'Sit until the Messenger of Allah, peace and blessings of Allah be upon him, comes.' When the Messenger of Allah, peace and blessings of Allah be upon him, came, the Ansari woman was shy to ask him, so she went out. Umm Salamah told the Messenger of Allah, peace and blessings of Allah be upon him, [about it]. He said: 'Call the Ansari woman.' She was called, and he recited to her this verse: 'Your wives are a place of cultivation for you, so come to your place of cultivation however you wish' [Al-Baqarah: 223], [meaning] in one passage."