Bukhari

Explanation:
From this it is understood that a person’s used water (saliva, etc.) is not impure. Just as the water from the mouth-rinsing (kuli) of the Prophet sallallahu alayhi wa sallam, regarding which he sallallahu alayhi wa sallam instructed them to drink it. From this, it is also understood that used water (ma’ musta’mal) is pure.

Hadith Commentary:
From this it is understood that a person's used water (saliva) is not impure (najis).
Just as the water from your (sallallahu alayhi wa sallam) rinsing of the mouth, which you (sallallahu alayhi wa sallam) instructed them to drink.
From this, it is also understood that used water (ma’ musta’mal) is pure.

Hadith Commentary:
➊
Imam Bukhari rahimahullah has narrated this report further on in a connected (mawsool) form (Sahih al-Bukhari, Book of Ablution, Hadith: 196), and in fact, in the Book of Maghazi, he has also mentioned its background.
Hazrat Abu Musa Ash‘ari radi Allahu anhu reports that the Messenger of Allah sallallahu alayhi wa sallam was encamped at the place of Ji‘irrana between Makkah and Madinah, and Hazrat Bilal radi Allahu anhu was with him.
I presented myself in his service.
An Arab Bedouin came to you and said:
“Why do you not give me something as you promised?” The Messenger of Allah sallallahu alayhi wa sallam, as encouragement, said:
(Abshir)
“Glad tidings for you.”
He replied:
“I do not want glad tidings, I want wealth.”
The face of the Messenger of Allah sallallahu alayhi wa sallam changed due to anger, and he said:
“He has rejected my glad tidings, so you two (Abu Musa Ash‘ari radi Allahu anhu and Bilal radi Allahu anhu),
accept it.”
They said:
“We accept it wholeheartedly.”
Then he called for a bowl in which there was water.
He washed his hands and face in it and rinsed his mouth.
Then he ordered Hazrat Abu Musa Ash‘ari radi Allahu anhu and Hazrat Bilal radi Allahu anhu to drink from it and to sprinkle it on their chests and faces, and he gave them glad tidings again.
They did so.
Meanwhile, Hazrat Umm Salamah radi Allahu anha spoke from behind the curtain, saying to save some water for her mother.
Thus, some water was set aside for Hazrat Umm Salamah radi Allahu anha.
(Sahih al-Bukhari, Maghazi, Hadith: 4328)
We have mentioned this background so that the scholarly inadequacy of those who claim “Tadabbur Qur’an” in their own estimation may become evident.
They only show disdain for the ahadith of Bukhari.
Thus, Islahi Sahib says:
“This is not part of the narration under discussion, but rather an addition to the narration.”
(Tadabbur Hadith 1/289)
Imam Bukhari rahimahullah mentioned this mu‘allaq (unattributed) narration after the hadith of Abu Juhaifah, not because it is part of the aforementioned hadith, but rather he narrated it as an independent hadith.
But Islahi Sahib intends to “correct” Bukhari, even if by an incorrect method.

➋
From this hadith, it is understood that used water (musta‘mal) can be reused; if it is permissible to drink it, then it can also be used for kneading dough and cooking food.
The Messenger of Allah sallallahu alayhi wa sallam washed his blessed hands and noble face in this bowl only for the sake of blessing (barakah), and for the same reason, he rinsed his mouth in it.
This ablution (wudu) of the Messenger of Allah sallallahu alayhi wa sallam was for the purpose of blessing; it was not a complete ablution, nor was it intended for prayer. Therefore, we clarified in the heading established by Imam Bukhari rahimahullah that there is latitude in this matter and it should not be restricted by imposing limitations from oneself.