كتاب النكاح
Wedlock, Marriage (Nikaah)
Chapter: Not to marry more than four (at a time)
باب لاَ يَتَزَوَّجُ أَكْثَرَ مِنْ أَرْبَعٍ
Sahih al-Bukhari 5098

Narrated Aisha":

(regarding) the Verse: 'And if you fear that you shall not be able to deal justly with the orphans...' (4.3) It is about the orphan girl who is in the custody of a man who is her guardian, and he intends to marry her because of her wealth, but he treats her badly and does not manage her property fairly and honestly. Such a man should marry women of his liking other than her, two or three or four. 'Prohibited to you (for marriage) are: ...your foster-mothers (who suckled you).' (4.23) Marriage is prohibited between persons having a foster suckling relationship corresponding to a blood relationship which renders marriage unlawful.

حَدَّثَنَا مُحَمَّدٌ، أَخْبَرَنَا عَبْدَةُ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، ‏{‏وَإِنْ خِفْتُمْ أَنْ لاَ، تُقْسِطُوا فِي الْيَتَامَى‏}‏‏.‏ قَالَتِ الْيَتِيمَةُ تَكُونُ عِنْدَ الرَّجُلِ وَهْوَ وَلِيُّهَا، فَيَتَزَوَّجُهَا عَلَى مَالِهَا، وَيُسِيءُ صُحْبَتَهَا، وَلاَ يَعْدِلُ فِي مَالِهَا، فَلْيَتَزَوَّجْ مَا طَابَ لَهُ مِنَ النِّسَاءِ سِوَاهَا مَثْنَى وَثُلاَثَ وَرُبَاعَ‏.‏
Reference : Sahih al-Bukhari 5098
In-book reference : Book 67, Hadith 36
USC-MSA web (English) reference : Vol. 7, Book 62, Hadith 35
  (deprecated numbering scheme)