פירוש על ברכות 4:13

F. Gemara: "From when do we begin to say the Shema in the evening? From the time that the day is sanctified on Erev Shabbat, these are the word of Rebbi Eliezer." See Rashi (DH M'sha'ah): We have by Bein Hashmashot (the time between sunset and the emergence of three medium sized stars, often translated as twilight, and literally means "between the watches") a doubt [whether it is] the day or [whether it is] the night, and since it is a doubt, the sanctification of the day is from a doubt." And this needs clarification, that Rashi reasons that the mitzvah of Kriyat Shema depends on Tzeiz HaKochavim (emergence of three medium starts which determines it is halachically night), and his opinion requires further exploration. How does Rebbi Eliezer reason that one has completed the Kriyat Shema of the evening, from [the time period of] sanctifying the day which is [in the period of] Bein Hashmashot which is in doubt [whether it is] night [or not]. And it appears that concerning many various laws, that after the sun sets, the status of 'night' is generated, and this can be found by blood that becomes invalid during the sunset, [so too] regarding Kriyat Shema is considered like night, that if there was a doubt of [whether or not it was] night, one would not be able to fulfill the mitzvah of Kriyat Shema [since] it is a doubt on a Torah level. In previous discussions, it was explained that by the setting of the sun, the timeframe of night begins [regarding] on a certain level like the opinion of the Gra, and it appears that Rebbi Eliezer reasons that the obligation of Kriyat Shema depends on the time[frame] of the night and not on the innate aspect of night [itself].

And it is agreed by all that the evening Shema' must not be read in the daytime. This question is dealt with below.