It is narrated from Sayyiduna Nu'man bin Bashir (may Allah be pleased with him) that his father gifted a slave to one of his sons, then brought him to the Prophet ﷺ to make him a witness to it. The Prophet ﷺ asked: "Have you gifted a slave to all your sons in this way?" The father replied, "No." So the Prophet ﷺ said: "Then take it back."
Hadith Referenceالسنن المأثورة / باب ما جاء في فدية الأذى / 494
Hadith Takhrijصحیح مسلم، الهبات، باب كراهة تفضيل بعض الاولاد فی الهبة، رقم: 1623۔