Rif Shabbat 3b

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3b

{Shabbat 9a continued}
Rav Yehuda cited Rav: he reference here is to the threshold of an alley, half of which [threshold] is covered and half uncovered, the covering being toward the inner side: [hence] if the door is open, it is as within; if the door is shut, it is as without.

{Shabbat 9b}

MISHNAH.
ONE MUST NOT SIT DOWN BEFORE A BARBER NEAR mincha UNTIL HE HAS PRAYED:
YET IF THEY BEGAN, THEY NEED NOT BREAK OFF.NOR MAY HE ENTER THE BATHS OR A TANNERY, NOR TO EAT NOR FOR A LAWSUIT,
ONE MUST BREAK OFF FOR THE READING OF THE SHEMA', BUT NOT FOR PRAYER.