Sahih Muslim Book 012, Hadith Number 3966.
Chapter : Disapproval of giving preference to tome children while making donation.
Nu'man b. Bashir reported that his mother bint Rawaha asked his (Nu'man's) father about donating some gifts from his property to his son. He deferred the matter by one year, and then set forth to do that. She (Nu'man's mother) said: I shall not be pleased unless you call Allah's Messenger (may peace be upon him) as witness to what you confer as a gift on your son. (Nu'man said): So father took hold of my hand and I was at that time a boy, and came to Allah's Messenger (may peace be upon him) and said: Allah's Messenger, the mother of this son (of mine), daughter of Rawaha wishes that I should call you witness to what I confer as gift to her son. Allah's Messenger (may peace be upon him) said: Bashir, have you any other son besides this (son of yours)? He said: Yes. He (the Holy Prophet) said: Have you given gifts to all of them like this? He said: No. Thereupon he (the Holy Prophet) said: Then call me not as witness, for I cannot be witness to an injustice.