مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3066
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
Abu Salamah bin ‘Abdur Rahman narrated that he heard Abu Hurairah say: Abu Bakr as-Siddeeq entered the mosque when 'Umar was speaking to the people... And he mentioned the same hadeeth.
Musnad Ahmad 3091Its isnad is saheeh]