Musnad Abu Ya'la al-Mawsili 151:54

Mis'ar said: Ma'n narrated to me, from Ja'far ibn 'Amr ibn Umayyah, from his father, from Ibn Mas'ud, who said: The Prophet (peace and blessings of Allah be upon him) said: "A witness over them as long as I am among them," or he said: "As long as I was among them." Mis'ar was in doubt.