Musnad Ahmad 652:17

Ya'la narrated to us, Abu Hayyan informed us, from al-Sha'bi, from al-Nu'man ibn Bashir, who said: My mother asked my father for some gift for me, so he gifted it to me. She said: "I will not be satisfied until you have the Messenger of Allah (peace and blessings of Allah be upon him) bear witness to it." He said: So my father took my hand while I was a boy and came to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "O Messenger of Allah, the mother of this one, the daughter of Rawahah, pressured me regarding some gift for him, and I have gifted it to him, and it pleased her that I have you bear witness." He said: "O Bashir, do you have a son other than this one?" He said: "Yes." He said: "Have you gifted to him the same as what you gifted to this one?" He said: "No." He said: "Then do not have me bear witness, for I do not bear witness to injustice."