Sahih Muslim Book 012, Hadith Number 3971.
Chapter : Disapproval of giving preference to tome children while making donation.
Jabir (Allah be pleased with him) reported that the wife of Bashir said (to her husband): Give to my son your slave as a gift, and make for me Allah's Messenger (may peace be upon him) a witness He came to Allah's Messenger (may peace be upon him) and said: The daughter of so and so (his wife Amra bint Rawaha) asked me to give my slave as a gift to her son, and call for me Allah's Messenger (may peace be upon him) as a witness. Thereupon he (the Holy Prophet) said: Has he (Nu'man) brothers? He (Bashir) said: Yes. He (further) said: Have you given to all others as you have given to him? He said: No. He said: Then it is not fair; and verily I cannot bear witness but only to what is just.