Sahih Bukhari Volume 007, Book 062, Hadith Number 071.
Narrated By 'Ursa bin Az-Zubair : That he asked 'Aisha, saying to her, "O Mother! (In what connection was this Verse revealed):
'If you fear that you shall not be able to deal justly with orphan girls (to the end of the verse) that your right hands possess?" (4.3) 'Aisha said, "O my nephew! It was about the female orphan under the protection of her guardian who was interested in her beauty and wealth and wanted to marry her with a little or reduced Mahr. So such guardians were forbidden to marry female orphans unless they deal with them justly and give their full Mahr; and they were ordered to marry women other than them." 'Aisha added, "(Later) the people asked Allah's Apostle, for instructions, and then Allah revealed: 'They ask your instruction concerning the women... And yet whom you desire to marry.' (4.127) So Allah revealed to them in this Verse that-if a female orphan had wealth and beauty, they desired to marry her and were interested in her noble descent and the reduction of her Mahr; but if she was not desired by them because of her lack in fortune and beauty they left her and married some other woman. So, as they used to leave her when they had no interest in her, they had no right to marry her if they had the desire to do so, unless they deal justly with her and gave her a full amount of Mahr."