Sahih Bukhari Volume 007, Book 062, Hadith Number 002.
Narrated By 'Ursa : That he asked 'Aisha about the Statement of Allah: 'If you fear that you shall not be able to deal justly with the orphan girls, then marry (other) women of your choice, two or three or four; but if you fear that you shall not be able to deal justly (with them), then only one, or (the captives) that your right hands possess. That will be nearer to prevent you from doing injustice.' (4.3) 'Aisha said, "O my nephew! (This Verse has been revealed in connection with) an orphan girl under the guardianship of her guardian who is attracted by her wealth and beauty and intends to marry her with a Mahr less than what other women of her standard deserve. So they (such guardians) have been forbidden to marry them unless they do justice to them and give them their full Mahr, and they are ordered to marry other women instead of them."