Sayyiduna Ja'dah bin Hubayrah (may Allah be pleased with him) said: Mention was made to the Prophet (peace and blessings be upon him) of a freed slave of Banu Abd al-Muttalib who would pray continuously and not sleep, and fast and not break his fast. He said: "I pray and I also sleep, I fast and I also break my fast. Every deed has a period of zeal, and every zeal is followed by a period of slackening. So whoever's slackening is toward the Sunnah has been guided, and whoever's is toward something else has gone astray."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2173