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Dharma Sutras 

According to Wikipedia: ’The Dharmasutras are Sanskrit texts dealing with custom, rituals and law. They include the four surviving written works of the ancient Indian tradition on the subject of dharma, or the rules of behavior recognized by a community. Unlike the later Dharmashastra, the dharmasutras are composed in prose. The oldest Dharmasutra is generally believed to have been that of Apastamba, followed by the dharmasutras of Gautama, Baudhayana, and an early version of Vasishtha. It is difficult to determine exact dates for these texts, but the dates between 500–300 BCE have been suggested for the oldest Dharmasutras.’