The tragic outcome of Shevet Ephraim's premature exodus from Egypt is referenced in several Jewish sources. Here are the key details:

1. According to midrashic tradition, some members of the tribe of Ephraim miscalculated the end of the 400 years of slavery prophesied to Abraham and left Egypt 30 years too early13.

2. This group of Ephraimites attempted to leave Egypt and enter Canaan on their own, without divine instruction13.

3. They were attacked and killed by the Philistines of Gath when they tried to enter Canaan prematurely135.

4. This tragic event is alluded to in 1 Chronicles 7:20-22, which describes sons of Ephraim being killed by the men of Gath135.

5. The Midrash connects this story to Exodus 13:17, explaining that God did not lead the Israelites by way of the land of the Philistines because the bones of the slaughtered Ephraimites were still visible there35.

6. Some sources state that 300,000 Ephraimites were killed in this failed attempt to leave Egypt early3.

7. This premature exodus attempt and its disastrous outcome is seen as a cautionary tale about trying to force the end of exile before the proper time decreed by God13.

So in summary, this tragic episode involving the tribe of Ephraim is found primarily in midrashic sources that expand on the biblical text, connecting it to verses in Chronicles and Exodus. It serves as an important lesson in Jewish tradition about patiently awaiting redemption according to God's timetable.