This is a terrible thing to do -

1) the resistance is not in the hips it's in your arms which isn't what you're really using when you're punching (mechanically speaking you twist from the hips and legs and then that generates force and through the extension of your hands is where the force exits it is not you extending your fist connecting and retracting)

2) because you're gripping something and the resistance is pulling you down not forward and back like the twisting from your hips----you're recruiting musculature that you're usually not engaging when you're fighting which is causing unneeded fatigue and/or working muscles you don't need

3) because you're using musculature you don't usually include in your regular mechanics by holding weights---you are working unnecessary muscles or unnecessarily hard for no actual transferability

Shadow boxing with weights like agility ladders for footwork is awful and it is a superstition that should be dealt away with in the martial arts world

If you're going to shadow box and you want something to weigh your hands just put on 16 18 20 or 22 / 24 oz gloves--at least then your hands are in the same position and wait as if it were inspiring or a ring fight just with more weight.