Musnad Ahmad 26:380

Aswad narrated to us, he said: And Khalaf ibn al-Walid informed us, Isra’il narrated to us, from Abu Ishaq, from Silah, from Ibn Mas’ud, who said: The ‘Aqib and the Sayyid, the two leaders of Najran, came. He said: They intended to invoke the curse of Allah (mula’anah) with the Messenger of Allah, peace and blessings of Allah be upon him. He said: One of them said to his companion: "Do not invoke the curse with him, for by Allah, if he is a prophet and we invoke the curse with him—Khalaf said: 'If we invoke the curse'—we will never succeed, neither we nor our descendants, ever." He said: So they came to him and said: "We will not invoke the curse with you, but we will give you what you asked for, so send a trustworthy man with us." The Prophet, peace and blessings of Allah be upon him, said: "I will surely send with you a trustworthy man, truly trustworthy, truly trustworthy." He said: The companions of Muhammad craned their necks for it. He said: Then he said: "Stand up, O Abu ‘Ubaydah ibn al-Jarrah." He said: When he turned to leave, he said: "This is the trustworthy one of this nation."