Charles Ellicott Commentary Numbers 35:22-23

"But if he thrust him suddenly without enmity, or hurled upon him anything without lying in wait, or with any stone, whereby a man may die, seeing him not, and cast it upon him, so that he died, and he was not his enemy, neither sought his harm;" — Numbers 35:22-23 (ASV)

But if he thrust him suddenly ... — See Deuteronomy 19:4-5, where the meaning of the law is illustrated.