Rif Shabbat 15b

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15b

BEGIN PEREK III
{Shabbat 36b}
MISHNA:
IF A [DOUBLE] STOVE {=a stove that held two pots} IS HEATED WITH STUBBLE OR RAKINGS, A POT MAY BE PLACED THEREON;
WITH PEAT OR WOOD, ONE MAY NOT PLACE [A POT THERE] UNTIL, HE SWEEPS IT OR COVERS IT WITH ASHES.
BETH SHAMMAI MAINTAIN: HOT WATER, BUT NOT A DISH {=food};
{Only the former may be placed there after it is swept; but not the latter}
BUT BETH HILLEL RULE; BOTH HOT WATER AND A DISH.

BETH SHAMMAI MAINTAIN: ONE MAY REMOVE [IT], BUT NOT PUT [IT] BACK; BUT BETH HILLEL RULE: ONE MAY PUT [IT] BACK TOO.

Gemara:
It was a question to them: Does this "ONE MAY NOT PLACE" {in the beginning of the Mishna} mean one must not put [it] back {on Shabbat if it were taken off}, yet it is permitted to keep [it there] {from erev Shabbat}