(18) بَابُ قَوْلِ اللَّهِ تَعَالَى: {وَلاَ تَنْكِحُوا الْمُشْرِكَاتِ حَتَّى يُؤْمِنَّ وَلأَمَةٌ مُؤْمِنَةٌ خَيْرٌ مِنْ مُشْرِكَةٍ وَلَوْ أَعْجَبَتْكُمْ}
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."
Reference: Sahih al-Bukhari 5285
In-book reference: Book 68, Hadith 34
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 63, Hadith 209