釋氏要覽 3.318

3.318

今有民俗。以辰飲一盃水。終日不食。謂之清齋。智度論云。劫初有聖人教人持齋。修善避凶。直以一日不食為齋。後佛出世。教人過中不食為齋。此為正法（言中者日午也。過午不得食）。

Nowadays there is a popular custom of drinking a cup of water at the chen hour (7–9 AM) and not eating for the rest of the day, calling it the "clear fast." The Mahāprajñāpāramitopadeśa says: "At the beginning of the kalpa, there was a sage who taught people to observe fasts, cultivate good, and avoid misfortune, simply taking not eating for a whole day as a fast. Later, when the Buddha appeared in the world, he taught people that not eating past noon was a fast; this is the true Dharma." ("Noon" refers to midday; one must not eat past noon.)