مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4052
Musnad of Abdullah bin Mas'ood
It was narrated from Abu Muhammad, the freed slave of 'Umar bin al-Khattab, from Abu 'Ubaidah bin ‘Abdullah... And he mentioned A similar report, except that he said: Abu Dharr said: I have only sent two ahead. This is also how Yazeed narrated it to us. He said: Abu Dharr said: I have sent two ahead.
Musnad Ahmad 4078Its isnad is da'eef because it is interrupted]