Oxidation is the losing of electrons.
SbH3 has Sb3+ ion in +3 oxidation state which changes to Sb which is in 0 oxidation state. It's gaining of electrons.
Now, Lets see (MnO4)-.
It has Mn7+ in +7 oxidation state which changes to MnO2 where Mn4+ is in +4 state. So, it gained electron.
I think it's Hydrogen. Because in SbH3, H is in -1 state which would change into H2 on product side. Not shown in question . But it should be there. Hydrogen gas will release. H2 will then have 0 state. Means loss of electron.
So, H atom is getting oxidised.
