Divorce whomever of them you wish

Good hadith

عن الضحاك بن فيروز، عن أبيه قال: قلت: يا رسول الله، إني أسلمت وتحتي أختان؟، قال: «طَلِّق أيتَهُما شِئتَ».

Ad-Dahhāk ibn Fayrūz reported from his father that he said: "I said: 'O Messenger of Allah, I have embraced Islam, and I have two sisters as wives!' He said: 'Divorce whomever of them you wish.'"

Explanation

Fayrūz ad-Daylami (may Allah be pleased with him) was a Companion from Yemen. He was married to two sisters when he embraced Islam. In the pre-Islamic period of ignorance, people used to combine two sisters in marriage without finding fault with that. Therefore, the Prophet (may Allah's peace and blessings be upon him) commanded Fayrūz to keep one of the two sisters as his wife and divorce the other. In Islam, it is not permissible to combine two sisters in marriage.

Source · Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad

From the Hadith Encyclopedia (hadeethenc.com).