Sayyidah Asma bint Yazid (may Allah be pleased with her) said: A drink was brought to the Messenger of Allah (peace and blessings be upon him), and it was passed around among the people. Among them was a man who was fasting. It was said: O Messenger of Allah, he fasts perpetually. So he (peace and blessings be upon him) said: "Whoever fasts perpetually has not fasted."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20824