Musnad Ahmad 27:1188

Abdullah ibn Ahmad said: Ibrahim ibn Sa'id narrated to us, Abu Usamah narrated to us, from 'Umar ibn Hamzah, from Salim, that a poet said in the presence of Ibn 'Umar: "And Bilal, the slave of Abdullah, is the best of Bilal." Ibn 'Umar said to him: "You have lied. That is Bilal, the Messenger of Allah's (may Allah bless him and grant him peace) Bilal."