Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him): "There is no li'an between four kinds of people: (1) a Christian or Jewish woman married to a Muslim, (2) a free woman married to a slave, (3) a slave woman married to a free man, (4) a Christian woman married to a Christian man."
Al-Shafi'i (may Allah have mercy on him) said: We said to them (those who cite it): You have narrated this from an unknown man and from a man who erred; and Amr bin Shu'ayb from Abdullah bin Amr is munqati' (broken). Of the two who narrated it, one attributes it to the Prophet (peace and blessings be upon him) and the other stops it at Abdullah bin Amr. So it is not established from Amr, nor from Abdullah bin Amr, and no one traces it back to the Prophet (peace and blessings be upon him) except a man who erred.
He said: Amr bin Shu'ayb has narrated to us from the Prophet (peace and blessings be upon him) rulings that agree with our positions and contradict yours; trustworthy narrators narrate them from him and trace them back to the Prophet (peace and blessings be upon him), yet you rejected them against us, rejected his narration and attributed it to error. So, if he is one whose hadith is established, you are refuted by his hadiths in about thirty rulings from the Prophet (peace and blessings be upon him) with which we agreed and which you opposed, most of which you opposed. And if he is one whose narration is not established, you are unfair when you use his narration as evidence, and then use from it as evidence something which, even if it were established from him and he were one whose hadith is established, we would not accept, because it is munqati' between him and Abdullah bin Amr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15296