Ishāq bin Mansūr said: I said:
Whoever prevents zakah should be fought? Ahmad said: Yes, Abū Bakr (may Allah be pleased with him) fought them till they performed that. He said: And whoever prevents an obligation, then it is a must upon the Muslims to fight him till they take that from him.
Ishāq said: Like he said, if they agree on that and get declare war.
“Masā’il al Kawsaj” (2343)