Midrash sa Exodo 21:13: Ein Yaakov (Glick Edition), Mekhilta DeRabbi Yishmael, Midrash Tanchuma Buber & Bereshit Rabbah

We are taught in a Baraitha that R. Eliezer b. Jacob said: (Ib. b) The word refuge was written at every crossing so that the murderer might recognize the way to take." Said R. Cahana: "Where is the passage to prove this? (Deut. 19, 3) Thou shalt prepare thee the way, i.e., you shall establish all preparations needed on this way." R. Chamma b. Chanina, when he wanted to lecture on this case, used to begin with (Ps. 25, 8) Good and upright is the Lord: therefore does He instruct sinners in the [right] way. Now if He puts the sinners in the right way, how much more the upright. Resh Lakish used to begin his lecture on this case with (Ex. 21, 13) And if a man lie not in wait, but God caused it to come to hand; and (1 Sam. 24, 14) As sayeth the proverb of the ancients: Out of the wicked cometh forth wickedness. Of what case does the passage speak? Concerning two men each of whom killed a person; one of them unintentionally [the punishment of which is exile] and the other intentionally [the punishment of which is death], but there were no witnesses in either case. The Holy One, praised be He! causes them to meet at the same inn, and he who had killed intentionally [guilty of capital punishment] is placed under a ladder, while the other, who killed unintentionally [and guilty of being exiled] descends the steps, falls and kills the other [under the ladder]. Hence the outcome is: He who has killed intentionally was killed; and the unintentional murderer was exiled.

(Exodus 21:13) "And if he did not lie in wait (for him) etc.": One killed unwittingly, and one killed wittingly (in the absence of witnesses). The second falls by the hand of the first.

Issi b. Akiva says: "then I shall make for you a place": What is the intent of this? (It is written in respect to the Sabbath boundary) (Exodus 16:29) "Let a man sit in his place" — two thousand ells.