This hadith was a major point of discussion during the First Fitna (civil war). It was used to identify which side was in the right during the Battle of Siffin.
Musnad Ahmad is a massive collection (over 30,000 entries), and numbering can vary between different print editions (e.g., Al-Maymani, Ar-Risalah). If you are looking at a specific print, let me know, as the text at "6929" might occasionally refer to a different narration about Apostasy or The Day of Judgment in other numbering systems. Musnad Ahmad - Sunnah.com - Sunnah.com musnad ahmad 6929 verified
, and snapped, "Why do you not keep this madman away from us? Why are you even with us if you believe this?" This hadith was a major point of discussion