Rashi on Numbers 35:23/רש"י על במדבר ל״ה:כ״ג

1או בכל אבן אשר ימות בה OR WITH ANY STONE WHEREBY ONE MAY DIE, he smote him.

2בלא ראות — i.e. that he (the slayer) did not see him (Not: without anyone having seen him, i.e. that there were no witnesses).

3ויפל עליו AND HE CAST IT UPON HIM — From here (i.e. from these words) they (the Rabbis) said: he who kills inadvertently by way of a falling movement goes into exile (i.e. must escape to and reside in a city of refuge); he who kills by way of an upward movement does not go into exile (Makkot 7b; cf. our Note on Exodus 21:13).