Kesubos (111a): Rab Judah stated in the name of Samuel: As it is forbidden to leave the Land of Israel for Babylon so it is forbidden to leave Babylon34 for other countries. Both Rabbah and R. Joseph said: Even from Pumbeditha35 to Be Kubi.36 A man once moved from Pumbeditha to [settle in] Be Kubi and R. Joseph placed him under the ban. A man once left Pumbeditha to [take up his abode at] Astunia,37 and he died.38 Said Abaye: ‘If this young scholar wanted it, he could still have been alive’.
Rambam(Melachim 5:12):... the prohibition is even to leave Babylonia for Israel
