Tosefta על דברים 15:9

Said Rabbi Yehoshuah ben Karha, "From where [do we learn] that everyone that turns his eyes away from [one seeking] charity is considered as if he worships idols (see Bava Batra 10a:10)? As it is written (Deut. 15:9), 'Beware that there be not a base thought in your heart... [and your eye be evil against your poor brother, and you give him nothing].' And moreover it states (Deut. 13:14), 'Certain base men have gone out […and have drawn away the inhabitants of their city, saying, Let us go and serve other gods].' Just as there, [the base men sin with] idolatry, so too here, [the base thought is treated like] idolatry" (Bava Batra 10a:10, following Steinsaltz).