Sahih Bukhari Volume 006, Book 060, Hadith Number 098.
Narrated By 'Urwa bin Az-Zubair : That he asked 'Aisha regarding the Statement of Allah:
"If you fear that you shall not be able to deal justly with the orphan girls..." (4.3) She said, "O son of my sister! An Orphan girl used to be under the care of a guardian with whom she shared property. Her guardian, being attracted by her wealth and beauty, would intend to marry her without giving her a just Mahr, i.e. the same Mahr as any other person might give her (in case he married her). So such guardians were forbidden to do that unless they did justice to their female wards and gave them the highest Mahr their peers might get. They were ordered (by Allah, to marry women of their choice other than those orphan girls." 'Aisha added," The people asked Allah's Apostle his instructions after the revelation of this Divine Verse whereupon Allah revealed:
"They ask your instruction regarding women " (4.127) 'Aisha further said, "And the Statement of Allah: "And yet whom you desire to marry." (4.127) as anyone of you refrains from marrying an orphan girl (under his guardianship) when she is lacking in property and beauty." 'Aisha added, "So they were forbidden to marry those orphan girls for whose wealth and beauty they had a desire unless with justice, and that was because they would refrain from marrying them if they were lacking in property and beauty."