The Messenger of Allah ﷺ was asked about water and the animals and predators that frequent it. The Messenger of Allah ﷺ replied, "When the water reaches two qullahs, it does not carry impurity."
سئل رسول الله ﷺ عن الماء وما ينوبه من الدواب والسباع، فقال رسول الله ﷺ: إذا كان الماء قلتين لم يحمل الخبث.
Abu Dawud (63, 64), Tirmidhi (67), Nasai (52), and Ibn Majah (517) all narrated it from Muhammad bin Jafar bin Zubayr, from Abdullah bin Abdullah bin Umar, from his father Abdullah bin Umar, except that Tirmidhi said, from Ubaydullah bin Abdullah bin Umar. Its chain is sahih, and Ibn Khuzayma (92), Ibn Hibban (1249), and Hakim (1/132-133) also graded it sahih. Hakim said, sahih according to the standard of the two Shaykhs, for they both relied on all its narrators but did not include it, and I think, and Allah knows best, that they did not include it because of a disagreement over it going back to Abu Usama, from Walid bin Kathir. End of quote.Ubaydullah, the diminutive form, and Abdullah, the full form, are both reliable narrators, and both transmit from their father Abdullah bin Umar. The kunya of Ubaydullah is Abu Bakr, and he was the full brother of Salim. The kunya of Abdullah is Abu Abdul-Rahman al-Madani. Ubaydullah died in the year 106 AH, and Abdullah died in the year 105 AH. For more, see Minna Kubra (1/278).As for his statement, "he frequents it," this refers to going to it again and again, turn after turn, from the verbs naba the place and intabahu meaning to visit repeatedly. As for the qullah, it is a vessel used by the Arabs like a large jar, and the jurists have estimated its capacity to be between two hundred and fifty and three hundred pounds.