Tosefta for Berakhot 4:4
והדר פשטו לה מברייתא מדקתני בברייתא סימן לדבר צאת הכוכבים שמע מינה ביאת שמשו הוא ומאי וטהר טהר יומא:
They then solved it from a Baraita ; for it is taught in a Baraita : The sign for the matter is the appearance of the stars. Conclude from this that the [complete] setting of the sun is intended ; and what means "It is clean" ? The day is clean.
Tosefta Berakhot
From when do we read Shema in the evenings? “From the time when people come [home] to eat their bread on Shabbat (Sabbath) nights,” [these are] the words of Rebbi Meir. And the Chachamim (Sages) say from the time when the Kohanim (priests) are able to eat their Terumah (heave offering). A sign for this is the coming out of the stars. And even though there is no explicit proof for this matter, there is an indirect reference: “… and half of them were holding spears from the morning twilight until the coming out of the stars.” (Nehemiah 4:15)
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