Midrash sur Samuel 2 3:38

He sought (to do) penitence, and spake before the Holy One, blessed be He: Sovereign of all worlds ! It is I who have sinned; forgive me, I beseech Thee, my sin. His repentance was accepted, and He said to the angel who had destroyed many (Rab) among the people: "Stay thine hand" (1 Chron. 21:15). What is the meaning of || "many" (Rab)? He said to him: Rab (the teacher) has fallen in Israel. What did the angel do? He took his sword and cleaned it with the garment of David. David saw the sword of the angel, and he trembled in all his limbs until his death (as it is said,) "But David could not go before it to inquire of God; for he was afraid because of the sword of the angel of the Lord" (1 Chron. 21:30).