Zur'ah bin Thawb said: I asked Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) about fasting perpetually, and he said: We used to count such people among us as being among the foremost. He said: I asked about fasting one day and leaving one day, and he said: That leaves the fasting person no room (for more). He said: And I asked him about the three fasts of every month, and he said: He has fasted the whole year and broken his fast.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8482