Sahih Muslim Book 12, Hadith Number 3961.

Chapter : Disapproval of giving preference to tome children while making donation.

Nu’man b. Bashir reported that his father brought him to Allah’s Messenger (may peace be upon him) and said: I have donated this slave of mine to my son. Allah’s Messenger (may peace be upon him) said: Have you donated to every one of your sons (a slave) like this? He said: No. Thereupon Allah’s Messenger (may peace he upon him) said: Then take him back.