Sifrei_mitzvot for Numbers 19:15

וְכֹל֙ כְּלִ֣י פָת֔וּחַ אֲשֶׁ֛ר אֵין־צָמִ֥יד פָּתִ֖יל עָלָ֑יו טָמֵ֖א הֽוּא׃

And every open vessel, which hath no covering close-bound upon it, is unclean.