Deuteronomy 19:5

As when a man goeth into the wood with his neighbour to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the head slippeth from the helve, and lighteth upon his neighbour, that he die; he shall flee unto one of those cities, and live:

Cross-reference

Numbers 35:25 outlines the congregation's role in protecting the unintentional killer, directly applying to the scenario in Deuteronomy 19:5.

Numbers 35:11 provides the original command for cities of refuge for unintentional killers, which the axe head example illustrates.

Numbers 35:22 gives the same law for accidental killing without enmity, providing the legal basis for the cities of refuge example here.