Musnad Ahmad 1156:159

Yazid ibn Harun narrated to us, and Hajjaj narrated to me, they both said: Ibn Abi Dhi'b informed us, from al-Maqburi, from Abdullah ibn Rafi', the freed slave of Umm Salamah, from Umm Salamah, that Umm Sulaym—Hajjaj said: the wife of Abu Talhah—said: "O Messenger of Allah, if a woman sees her husband in a dream having intercourse with her, is she required to perform ghusl (ritual bath)?" He said: "Yes, if she sees moisture." Umm Salamah said: "Does that happen?" He said: "May your right hand be covered in dust! From where else would the resemblance to the maternal uncles come if not from that? Whichever of the two fluids reaches the womb first, it prevails in the resemblance." And Hajjaj said in his narration: "May your forehead be covered in dust."