Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: The Prophet (peace and blessings be upon him) had a freed slave-woman who used to fast during the day and perform voluntary prayers at night. It was said to the Prophet (peace and blessings be upon him): "She fasts during the day and keeps performing voluntary prayers at night." The Prophet (peace and blessings be upon him) said: "Every deed has a period of zeal (that is, a person is energetic for it), and every zeal is followed by a lull (that is, an element of weakness or laziness). So whoever's lull leads towards my Sunnah will be rightly guided, and whoever's lull leads elsewhere will go astray."
This narration was recorded by Imam al-Bazzar, and its narrators are the narrators of the "Sahih".