Yes the Dacians used this type of weapon frequently and it is very similar to a scythe. This is from Wikipedia so take from that what you will, but I have read about it in Roman history books as well I just dont have them handy at this moment, but to quote wiki:
"The most important weapon of the Dacian arsenal was the falx. This
dreaded weapon, similar to a large sickle, came in two variants: a
shorter, one-handed falx called a sica,3 and a longer two-handed
version. The shorter falx was called sica (sickle) in the Dacian
language. The two-handed falx was a polearm. It consisted of a
three-feet long wooden shaft with a long curved iron blade of
nearly-equal length attached to the end. The blade was sharpened only
on the inside, and was reputed to be devastatingly effective. However,
it left its user vulnerable because, using a two-handed weapon, the
warrior could not also make use of a shield. Alternatively, it might
be used as a hook, pulling away shields and cutting at vulnerable
limbs."
Edit: After the OP added the condition of middle ages or later, I did find one source of the English Peasants Revolt of 1381 where the farmers/peasants armed themselves with axes, clubs, scythes, etc... and engaged royal forces. So this is one case they definitely used a scythe as you described as a weapon.
"On May 30, Thomas Bampton and two serjeants-at-arms went to Brentwood
to inquire into tax payments and to collect any money due. Thomas
Baker, the spokesman for Fobbing, informed him that they had already
paid and had a receipt from Bampton. They would not pay again. Bampton
became angry and threatened the men of Fobbing, but they had gathered
with the men from Corringham and Stanford: no one would pay. Bampton's
men tried to arrest the 100 men, but a mob drove -the three out of
Bampton and back to London to report the incident. There were
simultaneous uprisings on both sides of the names as peasants raised
their scythes, axes, and knives in this "battle worth
fighting...Between May 31 and June 1, the men of Essex gathered at
Brentwood, Baddow, and Colchester. By this time the news reached the
king, the revolt was under way."
source: Philip Lindsay and Reg Groves, The Peasants' Revolt 1381 (London: Hutchinson & Co., 1954) 20
link: English Peasants Revolt of 1381
Also found was the German Peasants Revolt of 1525
"While there were exceptions, the majority of conflicts were fought
with scythes, axes, flails, and other farming tools. The League was
also at an advantage by having armed horsemen, who could cover long
distances much faster than a marching peasant army. Altogether about
100,000 peasants and sympathizers either died in combat or were later
executed during the Peasant War of 1524–5. Many survivors were
tortured and lost their privileges and property. Cities and villages
that aided the revolts lost their rights; their weapons were
confiscated"
source: German Peasant Rebellion, 1525 by Simone Cezanne De Santiago Ramos