Reference for Pirkei Avot 5:23

בֶּן הֵא הֵא אוֹמֵר, לְפוּם צַעֲרָא אַגְרָא:

Ben Ha Ha says: According to the suffering is the reward. [The more "suffering" that you invest in toiling in Torah study and in doing mitzvoth, the greater will be your reward.]

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