וַיֹּ֨אמֶר מֹשֶׁ֜ה אֶֽל־אַהֲרֹ֗ן קַ֚ח צִנְצֶ֣נֶת אַחַ֔ת וְתֶן־שָׁ֥מָּה מְלֹֽא־הָעֹ֖מֶר מָ֑ן וְהַנַּ֤ח אֹתוֹ֙ לִפְנֵ֣י יְהוָ֔ה לְמִשְׁמֶ֖רֶת לְדֹרֹתֵיכֶֽם׃
And Moses said unto Aaron: ‘Take a jar, and put an omerful of manna therein, and lay it up before the LORD, to be kept throughout your generations.’
Rashi on Exodus
צנצנת is an earthern flask, as it is translated in the Targum (cf. Mekhilta d'Rabbi Yishmael 16:33).
Rashbam on Exodus
ויאמר משה אל אהרן, Moses said this to Aaron only after the Sanctuary had been built and inaugurated, i.e. at least about 11 months later. Until then there had not been a Holy Ark.