Al-Qasim narrates: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) bought a slave-girl from his wife - or from one of his wives - and laid down a condition for her, and the wife stipulated her service for herself. Then Sayyiduna Umar (may Allah be pleased with him) said: "That in which there is an exception for someone else does not fully become your property."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9168