Commentary for Berakhot 18:9
<big><strong>מתני׳</strong></big> מאימתי קורין את שמע בשחרית משיכיר בין תכלת ללבן ר' אליעזר אומר בין תכלת לכרתי (וגומרה) עד הנץ החמה ר' יהושע אומר עד שלש שעות שכן דרך מלכים לעמוד בשלש שעות
MISHNAH II From what time may the Shema' be read in the morning ? From the time that one can distinguish between blue and white. R. Eliezer says : Between blue and green. And he may finish it any time until sunrise. R. Joshua says : Until the third hour, for so is the custom of kings to rise at the third hour.
Abraham Cohen Footnotes to the English Translation of Masechet Berakhot
Similarly the Koran ii. 183 regards the day as commencing for the purpose of a fast when one "can plainly distinguish a white thread from a black thread by the daybreak."
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