Berachos 4a - Part 5, Berachos 4b - Part 1

In the Mishna the Chachamim said that one reads Shema until chatzos. The gemara asks:

Question: Who do the chachamim follow? If they follow R' Eliezer let them say like him? And, if the follow Rabban Gamliel let them say like him?

Answer: They actually follow Rabban Gamliel. They only say that you have until chatzos in order to distance one from aveirah. This is as we learned in a beraissa:

Beraissa: The Chachamim made a fence for their words in order that a person shouldn't come from the field in the evening and say let me eat, drink, and sleep a little and then I will say Shema and daven. Then he will be grabbed by sleep and it will turn out that he has slept all night. Rather one should come from the field in the evening and enter the shul - if he usually reads he should read, if he usually learns he should learn and he should read shema and daven and eat his bread and bentch. And anyone who transgresses the words of the sages is liable to death.

The Gemara asks on the Beraissa:

Question: Why only here does the beraissa specify the death penalty?

Answer # 1: Because there is the chance of accidentally falling asleep.

Answer # 2: We are trying to go against the one who says that davening maariv is a reshus. This beraissa comes to teach it is an obligation.