Apastambadharmasutra 2.3.6

If he has any doubts regarding the caste and conduct of a person who has come to him in order to fulfil his duty (of learning the Veda), he shall kindle a fire (with the ceremonies prescribed for kindling the sacrificial fire) and ask him about his caste and conduct.

If he declares himself to be (of) good (family and conduct, the teacher elect) shall say, 'Agni who sees, Vâyu who hears, Âditya who brings to light, vouch for his goodness; may it be well with this person! He is free from sin.' Then he shall begin to teach him.

A guest comes to the house resembling a burning fire.

He is called a Srotriya who, observing the law (of studentship), has learned one recension of the Veda (which may be current in his family).

He is called a guest (who, being a Srotriya), approaches solely for the fulfilment of his religious duties, and with no other object, a householder who lives intent on the fulfilment of his duties.

The reward for honouring (such a guest) is immunity from misfortunes, and heavenly bliss.

He shall go to meet such (a guest), honour him according to his age (by the formulas of salutation prescribed), and cause a seat to be given to him.

Some declare that, if possible, the seat should have many feet.

The (householder himself) shall wash the feet of that (guest); according to some, two Sûdras shall do it.

One of them shall be employed in pouring water (over the guest, the other in washing his feet).

Some declare that the water for the (guest) shall be brought in an earthen vessel.

But (a guest) who has not yet returned home from his teacher shall not be a cause for fetching water.

In case a (student comes, the host) shall repeat the Veda (together with him) for a longer time (than with other guests).

He shall converse kindly (with his guest), and gladden him with milk or other (drinks), with eatables, or at least with water.

He shall offer to his guest a room, a bed, a mattress, a pillow with a cover, and ointment, and what else (may be necessary).

(If the dinner has been finished before the arrival of the guest), he shall call his cook and give him rice or yava for (preparing a fresh meal for) the guest.

(If dinner is ready at the arrival of the guest), he himself shall portion out the food and look at it, saying (to himself), 'Is this (portion) greater, or this?'

He shall say, 'Take out a larger (portion for the guest).'

A guest who is at enmity (with his host) shall not eat his food, nor (shall he eat the food of a host) who hates him or accuses him of a crime, or of one who is suspected of a crime.

For it is declared in the Veda that he (who eats the food of such a person) eats his guilt.