Deuteronomy 19:13
Thine eye shall not pity him, but thou shalt put away the guilt of innocent blood from Israel, that it may go well
More from Deuteronomy 19
When the LORD thy God hath cut off the nations, whose land the LORD thy God giveth thee, and thou succeedest them, and dwellest in their cities, and in their houses;
Thou shalt separate three cities for thee in the midst of thy land, which the LORD thy God giveth thee to possess
Thou shalt prepare thee a way, and divide the coasts of thy land, which the LORD thy God giveth thee to inherit, into three parts, that every slayer may flee
And this is the case of the slayer, which shall flee thither, that he may live: Whoso killeth his neighbour ignorantly, whom he hated not in time past;
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: