Bukhari

Hadith Commentary:
➊
This is from the noble generosity of Allah Ta’ala that He forgives even major sins on account of a small good deed, provided that the deed is performed with sincerity. Thus, this sinful woman was forgiven due to her sincerity.
Furthermore, this hadith also shows that a Muslim who commits a major sin may have a small deed accepted, which becomes a means for his forgiveness.

➋
It should be noted that Imam Bukhari rahimahullah has also narrated a similar incident regarding a man, as reported from Abu Hurairah radi Allahu anhu.
(Sahih al-Bukhari, al-Musaqat, Hadith: 2363)
From these ahadith, it is understood that these are two separate incidents, and each has its own distinct status.

➌
Imam Bukhari rahimahullah has mentioned this hadith solely because it contains the mention of an animal.

Hadith Commentary:
It is understood from this that there is reward even in giving water to an animal.
This was the blessing of sincerity, that through a single good deed, that sinful woman was forgiven.

Hadith Commentary:
In another hadith, there is further clarification that she took off her scarf, tied it to her sock, and drew water from the well to give the dog to drink. Allah Ta’ala forgave her. (Sahih al-Bukhari, Book of the Beginning of Creation, Hadith: 3321)

A similar incident is also mentioned in a hadith regarding a man: he gave water to a thirsty dog, so Allah Ta’ala appreciated his deed and forgave him.

The noble Companions (radi Allahu anhum) asked:
“Is there a reward for us in serving animals as well?” He (sallallahu alayhi wa sallam) replied:
“There is a reward in serving every living being.” (Sahih al-Bukhari, Book of Agriculture, Hadith: 2363)

From these multiple incidents, it is understood that there is reward and merit even in giving water to an animal.

These mentioned incidents reveal the blessings of sincerity.

Hadith Commentary: Benefits and Issues: This hadith establishes that sometimes a seemingly minor good deed, when performed with sincerity and a spirit of compassion and goodwill, can transform a person’s entire life. The individual abandons wrongdoing and adopts the path of righteousness, thereby improving his Hereafter and causing his past sins to be erased. However, it is Allah alone who knows best which deed, at what time, becomes the cause of such a transformation or does not. Therefore, to use such ahadith as a basis for boldness and audacity in committing sins, or to consider sins as insignificant or trivial, is not a correct way of thinking.