“Leftover water” is what remains in a pot after some has been drunk. There are five different types of leftover water as follows:
Water leftover after people have drunk from the pot: is considered pure regardless of whether the one who drank from the pot was a Muslim, an unbeliever, a person in post-sex impurity or a menstruating woman.
Although Allah says in the Qur'an, "Verily, the idol worshippers are impure" (at-Taubah), this is a reference not to their physical state, but to their false beliefs and creed. They may come into contact with dirt or impurities, but this does not mean that their possessions or bodies are impure. In fact, they used to mix with the Muslims. Their emissaries and delegations used to visit the Messenger of Allah and enter his mosque. The Prophet (Sallallahou Alayhe Wasallam ﷺ ) did not order that the objects they touched be cleansed. As for menstruating women, 'Aishah said, "I used to drink (from a container) while I was menstruating. I would then pass it to the Messenger of Allah and he would drink from the same spot where I had put my lips." (related by Muslim.)
Water left in a container after an "allowable" animal (i.e., an animal whose meat is permissible to eat) has drunk from it: such water is considered pure. Since the animal qualifies for consumption, its saliva is also pure. Abu Bakr ibn al-Mundhir said, “The scholars are agreed that such water is permissible to drink or use for ablution."
Water remaining in a pot after it has been drunk from by a donkey, mule, beasts or birds of prey: such water is also considered pure, based on the hadith of Jabir in which the Messenger of Allah was asked about making ablution with drinking water left by donkeys. The Prophet (Sallallahou Alaihi Wasallam ﷺ ) answered, "Yes, and from the drinking water left by any of the beasts of prey." [this hadith was related by ash-Shafi, ad-Daraqutni and al-Baihaqi who said, "When its different chains are put together they become strong" but Shaykh al-Albani said: “This hadith is weak, as Al-Nawawi said in “Al-Majmoo’” 1/173”]
It has also been related from Ibn 'Umar that the Messenger of Allah went out at night while he was on a journey. He passed by a man who was sitting by a pond. Said "Umar, "Did a beast of prey drink from your pond tonight?" The Messenger of Allah (Sallallahou Alayhe Wasallam ﷺ ) told him, "O owner of the pond, do not inform him. It is not necessary, for him (the beast of prey) is what he carried in his stomach and for us is what he left, water to be used for drinking and purifying." [This is related by ad-Daraqutni. Al-Albani commented on this: "this is also weak according to ad-Daraqutni, 1/26 Ayoub bin Khaled Al-Harrani, al-Hafiz said he is “Weak” and he, despite his weakness, was confused in his chain of narration]
Yahya ibn Sa'eed reported that once Umar was among a group that included 'Amr ibn al-'Aas and, when they came upon a pond, 'Amr said, "O owner of the pond, have the beasts of prey discovered your pond?" Umar said, "Do not inform us, since the people drink after the wild beasts and the wild beasts after the people (Related by Imam Malik in al-Muwatta) [al-Albani said: this narration is from Yahya bin Abd al-Rahman on the authority of Umar, and not from the narration of Yahya bin Said on the authority of him, as said in the book. It was also narrated by al-Bayhaqi 1/250 via Malik and al-Daraqutni 1/22 via Hammad ibn Zayd: Yahya ibn Sa`id narrated it].
Water remaining in a pot after a cat has drunk from it: such water is also considered pure. This is proven by the hadith of Kabshah bint Ka'b who, when she was under the care of Abu Qatadah, entered the room to pour some water for him. A cat came, drank some of the water, and Qatadah proceeded to tilt the container so the cat could drink more. Kabshah said, "He noticed that I was watching him." He asked, "Are you surprised, O niece?" I answered, “Yes." He said, "The Messenger of Allah, upon whom be peace, said, It (the cat) is not impure. They intermingle with you." [Narrated by “the five” and al-Bukhari graded it as Sahih].
Water left in a pot after a pig or dog has drunk from it: such water is considered impure and must be avoided. Al-Bukhari and Muslim have recorded, on the authority of Abu Hurairah, that the messenger of Allah said, “If a dog drinks from one of your containers, wash it seven times." Ahmad and Muslim also have this addition, "Cleanse one of your containers if a dog licks it by washing it seven times, the first washing being with dirt." As for the leftover water of a pig, it is clearly considered filth and impure.