Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Apart from Ramadan, the Noble Prophet (peace and blessings be upon him) never fasted for an entire month, and when he would fast (voluntary fasts), he would continue fasting to the extent that someone would say, "By Allah! He will not stop fasting," and when he would stop (voluntary) fasting, someone would say, "By Allah! Now he will not fast at all."
Hadith Takhrijاس روایت کی سند صحیح اور حدیث متفق علیہ ہے۔ دیکھئے: [بخاري 1971] ، [مسلم 1157] ، [أبوداؤد 2430] ، [نسائي 2345] ، [ابن ماجه 1711] ، [أبويعلی 2602]
Brief Explanation
(Commentary on Hadith 1780)
From this hadith, the effort and hardship of the Messenger of Allah sallallahu alayhi wa sallam in worship and spiritual exercise is established, as well as his mercy and compassion for the Ummah. He himself used to fast, but for the ease of the Ummah, he would leave it so that a person may also fulfill the rights of his family and dependents, and not become solely devoted to prayer and fasting. For this reason, the Prophet sallallahu alayhi wa sallam also forbade perpetual fasting and would encourage his noble companions that your body has a right over you, your eyes have a right over you, your wife has a right over you, and while fulfilling all these rights, you should fast and worship.
Those who go out for spiritual retreats (chilla) for months at a time should reflect upon this hadith.