VaYikra 25:37

³⁷ אֶת־כַּסְפְּךָ לֹא־תִתֵּן לֹו בְּנֶשֶׁךְ וּבְמַרְבִּית לֹא־תִתֵּן אָכְלֶךָ׃

By Henry MacLagan

Verse 37. For it is unlawful to do good for the sake of reward, either from worldly or selfish motives.