Scythes do not have a stat block in 5th Edition D&D

Whether you’re a DM looking to issue a Scythe to a player, or a player trying to convince your DM to permit your character to use one, you’ll need to roll your own statblock for it; there’s neither a Mundane nor Magical Scythe to draw from.
My recommendation is to use the statblock for a Glaive or Halbard (which are the same)

The reasons for this are that it’s most likely to give you a weapon that resembles the classical manner in which a Scythe is used* in combat:

It will gain the Two-Handed property, matching the expectation that the weapon will be larger and wielded with two hands
It will gain the Reach property, which lets the user fight from a significant distance
It will gain the Heavy property, a companion property to the Two-Handed property that for many weapons will make them unwieldy for smaller characters

I would probably disallow gaining benefits from the Polearm Master feat though, since a Scythe is pretty unlikely to function the exact same as a Polearm, in terms of how it is swung.

* In Fantasy Fiction. In real life, Scythes are notoriously difficult and unfit to be used as weapons, and if D&D were attempting to recreate real life combat 1-to-1, Scythes would probably have a very poor statblock to capture that issue.