Shimon HaTzaddik was one of the last members of the Great Assembly. He used to say that the world endures because of three things: Torah, avodah (referring to the Temple service; nowadays, prayer), and gemilus chasadim (acts of kindness).
Antiginos of Socho received the Torah from Shimon HaTzaddik. He used to say not to act like servants who serve their master in order to receive a reward. Rather, we should act like servants who serve their master not in order to receive a reward, and we should let the “fear of Heaven” (i.e., awe of God) be upon us.