One historical document that relates to vegetarianism in Hinduism is the Manusmriti, which is a Hindu legal code. It is believed to have been written around 200 BCE to 200 CE, and it contains several references to vegetarianism.
In particular, the Manusmriti prescribes a vegetarian diet for Brahmins (priests) and other higher castes. It prohibits the killing of cows, which are considered sacred in Hinduism, and other animals for food. It also recommends the consumption of fruits, vegetables, grains, and milk products.
Here is an excerpt from the Manusmriti that emphasizes the importance of a vegetarian diet:
"Having well considered the origin of flesh-foods, and the cruelty of fettering and slaying corporeal beings, let one entirely abstain from eating flesh. Let him [the Brahmin] not eat what is obtained by hurting others, nor what has been kept for a night, nor what belongs to others, nor what has been given by others, nor what has been left over after a sacrifice, nor what has been kept in an unclean vessel, nor what is unripe, nor what is cooked by a woman in her courses, nor flesh of animals that have died a natural death." (Manusmriti 5.48-52)
This excerpt reflects the belief in Hinduism that all living beings are interconnected and that harming others, including animals, is detrimental to spiritual growth.