Mbh 13.106.4a

"'"Angiras said, 'As regards Brahmanas and Kshatriyas, fasts for three nights at a stretch are ordained for them, O delighter of the Kurus. Indeed, O chief of men, a fast for one night, for two nights, and for three nights, may be observed by them. (They should never go beyond three nights). As regards Vaisyas and Sudras, the duration of fasts prescribed for them is a single night. If, from folly, they observe fasts for two or three nights, such fasts never lead to their advancement. Indeed, for Vaisyas and Sudras, fasts for two nights have been ordained (on certain special occasions). Fasts for three nights, however, have not been laid down for them by persons conversant with and observant of duties. That man of wisdom who, with his senses and soul under control, O Bharata, fasts, by abstaining from one of the two meals, on the fifth and the sixth days of the moon as also on the day of the full moon, becomes endured with forgiveness and beauty of person and conversance with the scriptures. Such a person never becomes childless and poor. He who performs sacrifices for adoring the deities on the fifth and sixth days of the moon, transcends all the members of his family and succeeds in feeding a large number of Brahmanas. He who observes fasts on the eighth and the fourteenth days of the dark fortnight, becomes freed from maladies of every kind and possessed of great energy. The man who abstains from one meal every day throughout the month called Margasirsha, should, with reverence and devotion, feed a number of Brahmanas. By so doing he becomes freed from all his sins. Such a man becomes endued with prosperity, and all kinds of grain become his. He becomes endued with energy. In fact, such a person reaps an abundance of harvest from his fields, acquires great wealth and much corn. That man, O son of Kunti, who passes the whole month of Pausha, abstaining every day from one of two meals, becomes endued with good fortune and agreeable features and great fame.