Musannaf Ibn Abi Shayba 1:1714

Ibn ‘Uyaynah narrated to us, from ‘Ata’, who said: If a rat falls into a well, twenty buckets [of water] are to be drawn from it. If it has decomposed, then forty buckets. If a sheep falls into it, forty buckets are to be drawn from it. If it has decomposed, then all of it is to be drawn, or one hundred buckets.