Reference for Berakhot 3:8

תנא אקרא קאי דכתיב בשכבך ובקומך והכי קתני זמן קריאת שמע דשכיבה אימת משעה שהכהנים נכנסין לאכול בתרומתן

The Tanna bases his authority on Scripture ; for it is written, "When thou liest down and when thou risest up" (Deut. vi. 7). His statement in the Mishnah is to be understood thus : When is the time for reading the Shema' which is to be recited when lying down? From the time the priests enter [the Temple] to partake of their Terumah.

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