When Rabbi Akiva was in jail, Rabbi Yehoshua haGarsi would bring him water. Once the jailor poured out half the water, and Rabbi Akiva insisted on using what remained to wash his hands for his meal rather than to drink, saying, "What can I do? The punishment for violating the words of the rabbis is Death!": Eruvin 21b
Comparison between disdaining this Mitzvah and being involved in adultery: Sotah 4b
The punishment for disdaining this Mitzvah is poverty, or is being "uprooted" from the world: Sotah 4b