Anandakanda 1.4.27

1.4.27

kṣālayenmṛṇmaye pātre tataḥ soṣṇāranālakaiḥ /
mardanaṃ kṣālanaṃ caivamekaviṃśativāsaram //

Then one should wash it in an earthen vessel with warm fermented sour gruel; / Grinding and washing should be done for twenty-one days.