From Sahih Bukhari (Hadith 2660):
Narrated `Uqba bin Al-Harith:I married a woman and later on a woman came and said, "I suckled you both." So, I went to the Prophet (to ask him about it). He said, "How can you (keep her as a wife) when it has been said (that you were foster brother and sister)? Leave (divorce) her
Reference: Bukhari 2660