Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 197

להלוות לעני, שנאמר: "אם כסף תלווה את עמי" (שמות כב,כד); "אם" זה אינו רשות אלא מצוה, שנאמר: "והעבט, תעביטנו" (דברים טו,ח).

To lend to the poor, as [Exodus 22:24] states: "If you will lend money to my people...." In this instance, the word "if" does not refer to a matter left to one's volition, but to a commandment, as [Deuteronomy 15:8] states: "You shall surely lend him."