Al-Matalib al-'Aliyah 3196 — The virtue of persevering in deeds

It is narrated from Abu Fakhitah al-Taymi (may Allah have mercy on him) that a man came into the noble presence of the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah, my nephew (my sister's son) is exerting himself to the utmost in worship and has put himself into severe hardship (through excessive devotion)." The Messenger of Allah (peace and blessings be upon him) said: "This is the early zeal and fervour of Islam, and indeed every zeal and passion has a period of slackening and lethargy. So watch over him in his period of slackening: if he adopts moderation (even in the period of slackening), then there is hope that he will succeed; but if he is ruined (through excess and extremism), then destruction be upon him."
The narrator in the chain of this hadith, Abu Fakhitah, is thiqah (trustworthy), and supporting reports with this meaning are found in Musnad Ahmad (6477) and Sahih Ibn Hibban (11).