(Devarim 14:27) "And the Levite who is in your gate, you shall not forsake him" (by not giving him first tithe): Wherever you find this Levite, give him of his portion (first-tithe). If he does not have his portion, he gives him second-tithe. If he does not have second-tithe, he gives him poor-tithe. If he does not have poor-tithe, he gives him peace-offerings. If he does not have peace-offerings, he feeds him from charity, it being written (26) "you … (27) and the Levite." "for he has no portion or inheritance with you": This is the rationale (of the mitzvah).
Sifrei Devarim 108
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