Abu Dawud Book 24, Hadith Number 3939.

Chapter : Possession of a slave who is a relative to the master and is within the prohibited degrees.

Narated By Umar ibn al-Khattab : Qatadah reported Umar ibn al-Khattab (may Allah be pleased with him) as saying: If anyone gets possession of a relative who is within the prohibited degrees, that person becomes free.