Sahih Bukhari Volume 7, Book 67, Hadith 5066

حَدَّثَنَا عَلِيٌّ، سَمِعَ حَسَّانَ بْنَ إِبْرَاهِيمَ، عَنْ يُونُسَ بْنِ يَزِيدَ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عُرْوَةُ، أَنَّهُ سَأَلَ عَائِشَةَ عَنْ قَوْلِهِ تَعَالَى ‏{‏وَإِنْ خِفْتُمْ أَنْ لاَ تُقْسِطُوا فِي الْيَتَامَى فَانْكِحُوا مَا طَابَ لَكُمْ مِنَ النِّسَاءِ مَثْنَى وَثُلاَثَ وَرُبَاعَ فَإِنْ خِفْتُمْ أَنْ لاَ تَعْدِلُوا فَوَاحِدَةً أَوْ مَا مَلَكَتْ أَيْمَانُكُمْ ذَلِكَ أَدْنَى أَنْ لاَ تَعُولُوا‏}‏‏.‏ قَالَتْ يَا ابْنَ أُخْتِي، الْيَتِيمَةُ تَكُونُ فِي حَجْرِ وَلِيِّهَا، فَيَرْغَبُ فِي مَالِهَا وَجَمَالِهَا، يُرِيدُ أَنْ يَتَزَوَّجَهَا بِأَدْنَى مِنْ سُنَّةِ صَدَاقِهَا، فَنُهُوا أَنْ يَنْكِحُوهُنَّ إِلاَّ أَنْ يُقْسِطُوا لَهُنَّ فَيُكْمِلُوا الصَّدَاقَ، وَأُمِرُوا بِنِكَاحِ مَنْ سِوَاهُنَّ مِنَ النِّسَاءِ‏

Narrated By 'Urwa : 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."