Qurbani – Nafl Or Farz?
Not just Qurbani, Itikaf (seclusion) itself is transmitted through Sunnah but is not farz. Not everything which is part of religion necessarily needs to be farz too. There has always been ijma on its status as a nafli ibadah. Only Ahnaf consider it wajib (which is a term coined by Ahnaf and has no religious bearing). In effect, there has never been any disagreement on its staus as a Nafli Ibadah.
There was no tawaqquf in the ibadah of qurbani. It was also mandated for the bani Israel. https://en.m.wikipedia.org/wiki/Korban
i apologies if i am unable to understand your point, but just to clear this, what is evidence or proof of this ibadah of being a nafl?