From Sahih Bukhari (Hadith 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
Reference: Bukhari 5098