Leviticus 5:11-12

11A person may not have money to buy two doves or two pigeons. If he does not, he must give a tenth of an ephah (about two litres) of flour. He must not put oil or incense on the flour because it is a sin offering.

12He must take it to the priest. The priest will burn some of the flour on the altar as a special part of the flour. It is a sin offering to the Lord.