Musnad Ahmad 652:23

He said: And I heard An-Nu‘man ibn Bashir saying: My father, Bashir, gave me a gift. My mother said: "Call the Messenger of Allah (ﷺ) to witness it." So he took my hand and set off with me until we came to the Messenger of Allah (ﷺ). He said: "O Messenger of Allah, the mother of this boy asked me to give him a gift, so I gave it to him. She said: 'Call the Messenger of Allah (ﷺ) to witness it,' so I have come to you to call you as a witness." He (the Prophet) said: "Wait. Do you have children other than him?" He said: "Yes." He said: "Have you given all of them the same as you gave him?" He said: "No." He said: "Then do not call me to witness, for I do not bear witness to injustice. Your children have a right upon you that you should be just between them."