Tirmidhi

Explanation:
1:
This is evidence of Allah’s perfect mercy towards His servants.
It is due to this perfect mercy that Allah becomes extremely displeased with the misguidance or disobedient conduct of His servants,
and becomes extremely pleased when they repent, for now the servant has become deserving of My mercy.

Hadith Commentary: Benefits and Issues:
From this hadith, it is understood that the practice of the Prophet (sallallahu alayhi wa sallam) was not fixed and constant, but he would encourage his companions to fast on the "white days" (Ayyam al-Beedh), i.e., the 13th, 14th, and 15th of the lunar month. Therefore, if one is to fast only three days, then these are the most virtuous days to do so.

Benefits and Issues:
➊
The meaning of the hadith is that when a servant repents from sin, Allah Ta'ala becomes very pleased.

➋
When a servant realizes that he has committed a sin—whether it is a minor or major sin—he should repent directly to Allah. That is, by admitting his mistake and making a firm resolve and promise for the future that he will avoid that sin.

➌
Repentance is a matter between the servant and Allah. There is no need for a third party to intervene. However, admitting one's mistake before a righteous person and resolving to do good has the benefit that, at first, a person refrains from sin out of shame before that scholar, and then, by Allah's grace, he attains the ability to refrain from sin out of shame before Allah directly. Nevertheless, this is not necessary; if one repents in solitude and prays to Allah for steadfastness, that is sufficient.

➍
If the sin relates to the rights of people (huquq al-‘ibad), then in the case of committing it, it is necessary to fulfill that right or to seek forgiveness from the one entitled to it. Otherwise, the repentance will not be complete.