Wed Nazir 46

The Gemara says the Hava aminah of the Drasha to compare those without hand to people with hands regarding T'nufah, to tell us that if you don't do T'nufah, then you're not Yoitzeh

Tosfos explains: it's much more simpler to understand that people without hands are not Yoitzeh than people with hands but decided not to do it. We have the rule, any action that is not M'akeiv is only when you have the ability to do it. When you don't have the ability to do it, then it's M'akeiv and you're not Yoitzeh. Therefore, since someone without hands don't have the abilty to do T'nufah, so of course it's M'Akeiv, so too for people with hands T'nufah is M'akeiv.