Sahih Bukhari Volume 009, Book 086, Hadith Number 095.
Narrated By 'Urwa : That he asked 'Aisha regarding the Verse: 'If you fear that you shall not be able to deal justly with the orphan girls, marry (other) women of your choice.' (4.3) 'Aisha said, "It is about an orphan girl under the custody of her guardian who being attracted by her wealth and beauty wants to marry her with Mahr less than other women of her status. So such guardians were forbidden to marry them unless they treat them justly by giving them their full Mahr. Then the people sought the verdict of Allah's Apostle for such cases, whereupon Allah revealed: 'They ask your instruction concerning women...' (4.127) (The sub-narrator then mentioned the Hadith.)