Reference for Psalms 38:21

וּמְשַׁלְּמֵ֣י רָ֭עָה תַּ֣חַת טוֹבָ֑ה יִ֝שְׂטְנ֗וּנִי תַּ֣חַת רדופי־[רָֽדְפִי־] טֽוֹב׃

They also that repay evil for good are adversaries unto me, because I follow the thing that is good.

ספר השרשים