How does hydrogenation of ethylene show a reduction reaction?

The hydrogenation of ethylene is classified as a reduction reaction because it involves the transfer of electrons to the carbon atoms within the molecule.1

In this process, molecular hydrogen ($H_2$) reacts with ethylene. From the perspective of oxidation states, each of the two neutral hydrogen atoms in $H_2$ gives up an electron to the carbon skeleton. This electron transfer results in the reduction of the carbon atoms, while the hydrogen atoms themselves are oxidized, moving from an oxidation state of 0 to +1.1

Essentially, this reaction demonstrates that hydrogenation corresponds to a two-electron reduction of the two-carbon skeleton. By applying the oxidation-state concept, all such oxidation-reduction reactions can be formally viewed as electron-transfer processes.1