Sifrei Devarim 183

¹(Devarim 19:5) "and his hand swing the axe": From here it is ruled that if he intended to chop down a tree and it fell on a man and killed him, he is exiled. ²"and the blade slip from the wood ": from the haft (of the axe). Rebbi says: from the wood that was being chopped. ³"and it find his neighbor and he die": to exclude his presenting himself. From here R. Eliezer b. Yaakov ruled: If, after the stone left his hand, the other put out his head and was hit by it, the first is not liable. ⁴"he shall flee to one of these cities": He may not exile himself from one city to another. ⁵( Ibid . 6) "Lest the redeemer of blood pursue the slayer": It is a mitzvah for him to pursue him. ⁶"not having hated him": If he had hated him, he is not exiled. ⁷" mitmol shilshom ": As R. Yehudah says: " tmol ": two days; " shilshom ": three days (see #181). ⁸( Ibid . 7) "Therefore, I command you, saying": Beth-din is being exhorted to this end.