בְּטֶ֨רֶם אצורך [אֶצָּרְךָ֤] בַבֶּ֙טֶן֙ יְדַעְתִּ֔יךָ וּבְטֶ֛רֶם תֵּצֵ֥א מֵרֶ֖חֶם הִקְדַּשְׁתִּ֑יךָ נָבִ֥יא לַגּוֹיִ֖ם נְתַתִּֽיךָ׃
Before I formed thee in the belly I knew thee, And before thou camest forth out of the womb I sanctified thee; I have appointed thee a prophet unto the nations.
Rashi on Jeremiah
When I had not yet formed you in the womb, etc. Since the days of the first man. The Holy One, blessed be He, showed Adam each generation and its prophets.
Rashi on Jeremiah
I...formed you Heb. אצרך, an expression of צורה, a form.
Rashi on Jeremiah
I knew you connois toi in O.F. Comp. (Exodus 6:3), “I was not known (נודעתּי) to them.”