The association of reaping instruments - scythe or sickle - with Death can be traced back to the Bible, Revelation 14:14-16 (Rywiková, 2020):
14 And I looked, and behold, a white cloud; and upon the cloud sat one like unto the Son of Man, having on his head a golden crown and in his hand a sharp sickle.
15 And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, “Thrust in thy sickle and reap; for the time has come for thee to reap, for the harvest of the earth is ripe.”
16 And he that sat on the cloud thrust in his sickle on the earth, and the earth was reaped.
In Western art, the earliest depiction of personified Death holding a reaping instrument is found in the Uta Codex (Uta Codex Quattuor Evangelia), an evangeliary held at the Bavarian State Library, Munich, commissioned circa 1020-1025. It can be seen in the Crucifixion scene on folio 3v. A little decoding: Death, labelled mors in the mandorla (elliptic frame), is the standing male figure on the right, below Christ, turning away from the cross. The well-dressed character on the left is labelled vita, life, and he looks at Christ, unlike Death. Death is plainly dressed, his head is wrapped and he holds two objects: a broken spear (whose tip threatens his eye), a reminder of the spear that pierced Christ's side during the Crucifixion, and a broken sickle. There is a demonic figure next to him. The text says:
Mors devicta peris qui christum vincere gestis
You perish, conquered death, because you desire to conquer Christ (translation Cohen, 2000)
Here Death does not reap much and is not very triumphant - it has been "conquered" by Christ -, and the text echoes 1 Corinthians 15:55 "O death, where is thy victory ? O death, where is thy sting" (Cohen, 2000).
A more recent version (1414-1415, 400 years later!) version of that image can be found in the Biblia pauperum of the Metten Abbey, also in Bavaria: now Death looks like a smiling cadaver and still holds a broken spear, but it does not have a sickle or a scythe.
While the Bavarian Uta Codex shows a sickle-wielding but not very efficient Death, the first representations of our modern grim reaper only appear 300 years later, in Italy, in two paintings created in the same timeframe, circa 1335-1340. Now Death is triumphant and happy to harvest people:
In the 14th century, picturing death with a scythe became part of the iconography, first in Italy, and then in other European countries.
[The Triumph of Death](https://en.wikipedia.org/wiki/The_Triumph_of_Death_(Palermo), Palermo, c. 1440-1445, where Death wields a bow but wears a scythe on its saddle.
The Triumph of Death, Flemish school, c. 1510
The Triumph of Death, Pieter Bruegel the Elder, c. 1562
Note that one mythological figure typically associated with the scythe is Saturn, as god of harvest (Brumble, 1998). But Saturn is also associated with Time, and representations of Saturn devouring his children sometimes include the scythe, either as an attribute of Saturn himself (Giovanni Jacopo Caraglio, after Rosso Fiorentino, 1526) or as an attribute of Death as a follower of Saturn, as in Bruegel's [Triumph of Time] (https://www.nga.gov/collection/art-object-page.47624.html) (1574).
Sources