Sayyiduna Abdullah bin Amr (may Allah be pleased with him) says: Mention was made before the Prophet (peace and blessings be upon him) of some people who worshipped with great exertion. The Prophet (peace and blessings be upon him) said: "This is the need of Islam and its vigour (that is, being energetic), and every vigour is followed by slackness. So whoever's leaving off (of deeds due to slackness) leads towards moderation, that is a good thing; and whoever's leaving off (of deeds due to slackness) leads towards sin, those are the ones who are ruined."
This narration was recorded by Imam al-Tabarani in al-Mu'jam al-Kabir, and Imam Ahmad recorded a similar narration. The narrators of Imam Ahmad are "thiqat" (trustworthy). Ibn Ishaq stated: "Abu al-Zubayr narrated the hadith to me," so in this way the tadlis (concealment in the chain) is removed.