Narrated Nafi':

Whenever Ibn 'Umar was asked about marrying a Christian lady or a Jewess, he would say: "Allah has made it unlawful for the believers to marry ladies who ascribe partners in worship to Allah, and I do not know of a greater thing, as regards to ascribing partners in worship, etc. to Allah, than that a lady should say that Jesus is her Lord although he is just one of Allah's slaves."

حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا لَيْثٌ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، كَانَ إِذَا سُئِلَ عَنْ نِكَاحِ النَّصْرَانِيَّةِ، وَالْيَهُودِيَّةِ، قَالَ إِنَّ اللَّهَ حَرَّمَ الْمُشْرِكَاتِ عَلَى الْمُؤْمِنِينَ، وَلاَ أَعْلَمُ مِنَ الإِشْرَاكِ شَيْئًا أَكْبَرَ مِنْ أَنْ تَقُولَ الْمَرْأَةُ رَبُّهَا عِيسَى، وَهْوَ عَبْدٌ مِنْ عِبَادِ اللَّهِ.