Musnad Ahmad 3905
Musnad of Abdullah bin Mas'ood
It was narrated that Ibn Mas'ood (رضي الله عنه) said: al-'Aqib and as-Sayyid, the leaders of Najran, came and wanted to engage in mula'anah (mutual invoking of curses upon whichever party is wrong) with The Messenger of Allah (ﷺ). Then one of them said to the other: Do not engage in mula'anah with him, for by Allah if he is a Prophet (ﷺ) and engages in mula'anah with us, we will never prosper and neither will our offspring. Then they went to him and said: We will not engage in mula'anah with you; rather we will give you what you asked for. Send with us a trustworthy man. And the Prophet (ﷺ) said: "I shall certainly send a trustworthy man; he is indeed trustworthy, he is indeed trustworthy.” All the Companions of Muhammad (ﷺ) hoped to be the one, then he said: "Get up, O Abu Ubaidah bin al-Jarrah.” When he left with them, he (the Prophet (ﷺ) said: “This is the trustee of this ummah."