Muwatta Malik

Yahya related to me from Malik from Yahya ibn Said that Abu Nahshal ibn al-Aswad said to al-Qasim ibn Muhammad,""I saw a slave-girl of mine uncovered in the moonlight, and so I sat on her as a man sits on a woman. She said that she was menstruating, so I stood up and have not gone near her after that. Can I give her to my son to have intercourse with?"" Al-Qasim forbade that