Reference for Numbers 19:9

וְאָסַ֣ף ׀ אִ֣ישׁ טָה֗וֹר אֵ֚ת אֵ֣פֶר הַפָּרָ֔ה וְהִנִּ֛יחַ מִח֥וּץ לַֽמַּחֲנֶ֖ה בְּמָק֣וֹם טָה֑וֹר וְ֠הָיְתָה לַעֲדַ֨ת בְּנֵֽי־יִשְׂרָאֵ֧ל לְמִשְׁמֶ֛רֶת לְמֵ֥י נִדָּ֖ה חַטָּ֥את הִֽוא׃

And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place, and it shall be kept for the congregation of the children of Israel for a water of sprinkling; it is a purification from sin.