It was narrated from Samurah bin Jundub that the Prophet (ﷺ) said:
"Whoever becomes the master of a Mahram relative (with whom marriage is not lawful), he becomes free." (RU)
English reference (RU): Vol. 3, Book 19, Hadith 2524 (RU)
Arabic reference (RU): Book 19, Hadith 2620 (RU)