"The Prophet heard that I was fasting continually," and he quoted this Hadith. 'Ata said: "I am not sure how he put it, but I think he said: 'There is no fast for one who fats continually." 'Ata said: "someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted."
The Book of Fasting - Sunan an-Nasa'i 2378
In the name of Allah, the Most Gracious, the Most Merciful. This narration from Sunan an-Nasa'i addresses the important matter of continuous fasting without break. The Prophet Muhammad (peace be upon him) cautioned against excessive worship that goes beyond the moderate path prescribed by Islamic law.
Scholarly Commentary
The prohibition against perpetual fasting (wisal) demonstrates Islam's balanced approach to worship. The scholars explain that such continuous fasting harms the body, weakens one's ability to fulfill other obligations, and contradicts the Sunnah which encourages moderation.
Imam An-Nawawi states in his commentary that the phrase "he has not fasted" means his fasting is not considered complete or proper in the sight of Allah, as it violates the Prophetic guidance. The wisdom behind this prohibition includes preserving health, maintaining strength for other acts of worship, and avoiding extremism in religion.
The legitimate fast is that which follows the Prophetic example - fasting some days and breaking fast on others, as in the case of fasting Mondays and Thursdays, or the three white days each month, while ensuring the obligatory fast of Ramadan is properly observed.
Legal Ruling
The consensus of scholars holds that continuous fasting for multiple days without breaking the fast at night is prohibited (haram). The Hanafi, Maliki, Shafi'i and Hanbali schools all agree on this ruling based on this and other authentic narrations.
Ibn Qudamah in Al-Mughni explains that this prohibition applies even if one feels capable of continuing the fast, because the harm is not merely physical but includes neglecting the Sunnah of breaking fast and potentially leading to religious innovation.