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." (ID)
English reference (ID): Vol. 3, Book 19, Hadith 2524 (ID)
Arabic reference (ID): Book 19, Hadith 2620 (ID)