Sahih Muslim Book 12, Hadith Number 3962.

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

Nu’man b. Bashir reported: My father brought me to Allah’s Messenger (may peace be upon him) and said: I have donated this slave to my son. whereupon he said: Have you made (such) donation to every one or your sons? He said: No. Thereupon he (the-Holy Prophet) said: Then take him back.