Sahih Muslim Book 012, Hadith Number 3970.
Chapter : Disapproval of giving preference to tome children while making donation.
Nu'man b. Bashir reported: My father conferred a gift upon me, and then brought me to Allah's Messenger (may peace be upon him) to make him a witness (to it). He (the Holy Prophet) said: Have you given such gift to every son of yours (as you have given to Nu'man)? He said: No. Thereupon he (the Holy Prophet) said: Don't you expect goodness from them as you expect from him? He said: Yes. of course. He (the Holy Prophet) said: I am not going to bear witness to it (as it is injustice). Ibn Aun (one of the narrators) said: I narrated this hadith to Muhammad (the other narrator) who said: Verily we narrated that lie (the Holy Prophet) had said: Observe equity amongst your children.