(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."
(דברים יד כז) וְהַלֵּוִי אֲשֶׁר בִּשְׁעָרֶיךָ, כָּל מָקוֹם אַתָּה מוֹצֵא לֵוִי הַזֶּה לוֹמַר: תֵּן לוֹ מֵחֶלְקוֹ! אֵין לוֹ חֶלְקוֹ – תֵּן לוֹ מַעְשַׂר עָנִי! אֵין לוֹ מַעְשַׂר עָנִי – תֵּן לוֹ שְׁלָמִים! אֵין לוֹ שְׁלָמִים – פַּרְנְסֵהוּ מִן הַצְּדָקָה!
(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."
כִּי אֵין לוֹ חֵלֶק וְנַחֲלָה עִמָּךְ, לְהַגִּיד מֶה גָּרַם.
"for he has no portion or inheritance with you": This is the rationale (of the mitzvah).