Majma al-Zawaid 5165 — Perpetual fasting

Mujahid narrated: Yahya bin Ja'dah and I went to a Companion from the Ansar. He narrated: A freed slave woman of Banu Abd al-Muttalib was mentioned before the Prophet (peace and blessings be upon him), and the speaker said: "She spends the whole night in voluntary prayer and fasts during the day." The Prophet (peace and blessings be upon him) said: "But I sleep (at night) and I also pray, and I also leave (voluntary) fasting. Whoever follows me is from me, and whoever turns away from my Sunnah has nothing to do with me. Every deed has a period of zeal, then slackness comes into it. Whoever's slackness leads to innovation (bid'ah) has gone astray, and whoever's slackness leads to my Sunnah has been guided."
Imam Ahmad recorded it, and its narrators are the narrators of the Sahih. Some hadiths similar to this have already passed.