רְאוּבֵן֙ בְּכֹ֣רִי

"to the east of Eden: There his father was exiled when he was driven out of the Garden of Eden, as it is said (3:24) 'and He stationed at the east of the Garden of Eden, etc., to guard' the way of approach to the Garden, from which we can learn that Adam was there. And we find that the easterly direction always offers asylum for murderers, as it is said (Deut. 4:41): 'Then Moses separated, etc. [three cities of refuge] in the direction of the sunrise' - [Mid. Devarim Rabbah, Lieberman, p.60; Tan. Buber ad loc.]. Another explanation: בְּאֶרֶץ נוֹד means that wherever he went, the earth would quake beneath him, and the people would say, 'Go away from him; this is the one who killed his brother' [Mid. Tan., Bereishith 9]." ~ Rashi