Sahih Muslim Book 8, Hadith Number 3274.

Chapter: It is forbidden to combine a woman and her father’s sister or a woman and her mother’s sister together in marriage.

Abu Huraira (Allah be pleased with him) reported Allah’s Apostle (May peace be upon him) as saying: A man must not make proposal of marriage to a woman when his brother has done so already. And he must not offer a price for a thing for which his brother had already offered a price; and a woman must not be combined in marriage with her father’s sister, nor with her mother’s sister, and a woman must not ask to have her sister divorced in order to deprive her of what belongs to her, but she must marry, because she will have what Allah has decreed for her.