Sahih Muslim | Book 43 | Hadith Number 7158
Narrated by 'Aisha
'Aisha said that as for the words of Allah: "If you fear that you would not be able to observe equity in case of orphan girls)," it was revealed in reference to a person who had an orphan girl (as his ward) and he was her guardian, and her heir, and she possessed property, but there was none to contend on her behalf except her own-self. And he (her guardian) did not give her in marriage because of her property and he tortured her and ill- treated her, it was in relation to her that (Allah said:) "If you fear that you would not be able to observe equity in case of orphan girls, then marry whom you like among women," i.e. whatever I have made lawful for you and leave her whom you are putting to torture.