“Antignos of Socho received the tradition from Shim’on the Righteous. He would say: Do not be as servants, who serve their master for the sake of reward. Rather, be as servants who serve their master not for the sake of reward. And the fear of Heaven should be upon you” (M Avot 1:3).
“Rabbi [Yehudah ha-Nasi] would say: Be as careful with a minor mitsvah as with a major one, for you do not know the rewards of the mitsvot. Consider the cost of a mitsvah against its rewards, and the rewards of a transgression against its cost” (M Avot 2:1).
“Son of Heh-Heh said: According to the effort is the reward” (M Avot 5:26).
“Many masters of study, colleagues of yours, nightly cry out contradictions in the Oral Torah, crying like dogs who say ‘hav! hav!’, as is written: The leech has two daughters, הַב הַב (hav hav), ‘give’ and ‘give’ (Proverbs 30:15) [cf. BT Avodah Zarah 17a, ad loc.]. Give us wealth in this world, give us wealth in the next world, as it is taught: ‘Learn much Torah and you will be given much reward’ (M Avot 2:16). But there is no one who is occupied with Torah in order to raise the Shekhinah, from the exile and unite Her with the Husband, for their eyes are blind, and their hearts are sealed” (Tiqqunei ha-Zohar 97c).