Avos 1:2

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).

Avos 1:3

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.