Musnad Ahmad 652:64

Sufyan narrated to us, Mujalid narrated to us, he said: I heard al-Sha'bi, he said: I heard al-Nu'man ibn Bashir saying—and he was the governor of Kufa—: My father gave me a slave as a gift, so I came to the Prophet, peace and blessings of Allah be upon him, to have him witness it. He said: "Have you given all of your children a gift like this?" He said: "No." He said: "Then I will not bear witness to an injustice."