Yahya related to me from Malik from Yahya ibn Said that Said ibn al-Musayyab said, "It is forbidden to be married to a woman and her paternal or maternal aunt at the same time, and for a man to have intercourse with a female slave who is carrying another man's child."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ كَانَ يَقُولُ يُنْهَى أَنْ تُنْكَحَ الْمَرْأَةُ عَلَى عَمَّتِهَا أَوْ عَلَى خَالَتِهَا وَأَنْ يَطَأَ الرَّجُلُ وَلِيدَةً وَفِي بَطْنِهَا جَنِينٌ لِغَيْرِهِ ‏.‏
USC-MSA web (English) reference : Book 28, Hadith 21
Arabic reference : Book 28, Hadith 1115