Musnad Ahmad 1034:156

'Affan narrated to us, Shu'bah narrated to us, he said: Abu Ishaq informed us, he said: I heard Silah ibn Zufar, from Hudhayfah, that the Messenger of Allah (peace and blessings of Allah be upon him) said to the people of Najran: "I will certainly send to you a man who is trustworthy, truly trustworthy." He said it more than twice. The people looked forward to it, so he sent Abu 'Ubaydah.