Sayyiduna Ja'dah bin Hubayrah (may Allah be pleased with him) says: Mention was made before the Prophet (peace and blessings be upon him) of a freed slave of Banu Abd al-Muttalib who kept praying and did not sleep, and kept (voluntary) fasting and never left a fast. The Prophet (peace and blessings be upon him) said: "I perform (voluntary) prayer and I also sleep; I keep (voluntary) fasts and I also leave them. Every deed has a period of vigour (that is, an aspect of being energetic), and every vigour is followed by weakness (and an aspect of tiredness). So whoever's weakness (that is, the aspect of leaving the deed) leads towards the Sunnah will be rightly guided, and whoever's leads towards something other than that (Sunnah) will go astray."
This narration was recorded by Imam al-Tabarani in al-Mu'jam al-Kabir, and its narrators are the narrators of the "Sahih".