The description of the warding bond spell says, in part (emphasis mine):
This spell wards a willing creature you touch and creates a mystic connection between you and the target until the spell ends. While the target is within 60 feet of you, it gains a +1 bonus to AC and saving throws, and it has resistance to all damage. Also, each time it takes damage, you take the same amount of damage.
The description of the hellish rebuke spell says:
You point your finger, and the creature that damaged you is momentarily surrounded by hellish flames. The creature must make a Dexterity saving throw. It takes 2d10 fire damage on a failed save, or half as much damage on a successful one.
When casting hellish rebuke, is the creature that receives the damage the one that cast warding bond? Or the one that attacked the creature that cast warding bond?
Both could be said to have damaged you, since the damage could not exist without both of them. Warding bond is the spell that actually gives you the damage, but it was cast before the damage was taken, and the attacking creature is the last one that creates a damaging action.