Sahih Bukhari Volume 007, Book 062, Hadith Number 010.
Narrated By Anas bin Malik : 'Abdur-Rahman bin 'Auf came (from Mecca to Medina) and the Prophet made a bond of brotherhood between him and Sad bin Ar-Rabi' Al-Ansari. Al-Ansari had two wives, so he suggested that 'Abdur-Rahman take half, his wives and property. 'Abdur-Rahman replied, "May Allah bless you with your wives and property. Kindly show me the market." So 'Abdur-Rahman went to the market and gained (in bargains) some dried yoghurt and some butter. After a few days the Prophet saw Abdur-Rahman with some yellow stains on his clothes and asked him, "What is that, O 'Abdur-Rahman?" He replied, "I had married an Ansari woman." The Prophet asked, "How much Mahr did you give her?" He replied, "The weight of one (date) stone of gold." The Prophet said, "Offer a banquet, even with one sheep."