Sahih Muslim Book 012, Hadith Number 3967.
Chapter : Disapproval of giving preference to tome children while making donation.
Nu'man b. Bashir, reported that Allah's Messenger (may peace be upon him) had said: Have you, besides him, other sons? He said: Yes. Thereupon he (the Holy Prophet) said: Have you given gifts to all of them like this (as you have given to Nu'man)? He said: No. Thereupon he (the Holy Prophet) said: I cannot bear witness to an injustice.