Muwatta Malik, Book 28, Hadith 1106 (In-Book)

Yahya related to me from Malik from Ibn Shihab that Zayd ibn Thabit said, "When a man takes his wife to his house and co-habits with her then the bride-price is obliged."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّ زَيْدَ بْنَ ثَابِتٍ، كَانَ يَقُولُ إِذَا دَخَلَ الرَّجُلُ بِامْرَأَتِهِ فَأُرْخِيَتْ عَلَيْهِمَا السُّتُورُ فَقَدْ وَجَبَ الصَّدَاقُ ‏.‏

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also referenced under: Book 28, Hadith 13 (USC-MSA)