Musnad Abu Ya'la al-Mawsili 149:39

Suwayd ibn Sa'id narrated to us, Ibn Abi Za'idah narrated to us, from his father, from Mus'ab ibn Shaybah, from Musafi' ibn 'Abd Allah, from 'Urwah ibn al-Zubayr, from 'Aishah: That a woman said to the Prophet, may Allah bless him and grant him peace, "Does a woman perform ghusl (ritual bath) if she has a wet dream and sees the fluid?" He said, "Yes." 'Aishah said to her, "May your hands be covered in dust!" The Prophet, may Allah bless him and grant him peace, said, "Leave her. And is the resemblance [of the child] except from that? If her fluid overcomes the man's fluid, the child resembles his maternal uncles, and if the man's fluid overcomes the woman's fluid, he resembles him."