Darimi

Urdu footnote:
“He established a good practice.” This is on the condition that the act is already present in the Shari‘ah, as in the above incident where the Ansari initiated a righteous deed and, seeing him, others also gave charity. And charity (sadaqah) is legislated in the Shari‘ah. If someone establishes a practice that is not present in the Shari‘ah, then this will be an innovation (bid‘ah), even if outwardly it appears to be a good deed, because the Messenger of Allah (sallallahu alayhi wa sallam) said: “Whoever introduces into this religion of ours something that is not from it, it is rejected.” (Sahih al-Bukhari, al-Sulh, Hadith: 2697, and Sahih Muslim, al-Aqdiyah, Hadith: 1718) Because in this way, distortion would enter the religion and the original form and appearance of the religion would not remain intact.

Explanation:
1:
From this hadith, it is understood that the reward for calling towards guidance—that is, faith (iman), monotheism (tawhid), and adherence to the Sunnah—and for spreading it, is such that not only will the person receive the reward for his own action, but in addition, he will continue to be granted a reward equal to that of all those who act upon it until the Day of Judgment.
Similarly, those who initiate evil, corruption, bloodshed, and innovations (bid‘ah) and superstitions will not only bear the sin of their own deeds, but also the burden of the sins of all those who act upon them.

Commentary:

➊
The literal meaning of "sunnah" is "way" or "method," whether good or bad.
In this hadith, the word is used in its literal sense.
In the sciences of hadith and the principles of hadith, "sunnah" includes everything narrated from the Messenger of Allah sallallahu alayhi wa sallam, whether it is his statement (qawli hadith),
his action (fa’li hadith),
or something done in his presence and, after becoming aware of it, he did not prohibit or reject it (taqreeri hadith).
The jurists (fuqaha) mean by "sunnah" a good deed that is neither obligatory (fard) nor necessary (wajib); this is also called "mustahabb" (recommended). Furthermore, the word "sunnah" is also used in contrast to "bid‘ah" (innovation), meaning a belief or action whose obligation, recommendation, or permissibility is established by the Shari‘ah, while "bid‘ah" refers to an act done with the belief of reward, even though it has no basis in the Shari‘ah.

➋
To establish a good way means that there was a need for a good deed and no one was doing it.
Or, a sunnah had been abandoned, and someone revived it, so seeing him, others also began to act upon it.
Or, to promote a legislated (mashru‘) act, a new method was adopted. Merely declaring something good based on one’s own opinion and inventing it is "bid‘ah," for which there is sin instead of reward.

➌
To establish an evil way means that an evil which did not previously exist in a society was initiated by a person, and seeing him, others also began to do that act.
The Prophetic statement is:
"Whoever soul is killed unjustly, a portion of its unjust blood is upon Adam alayhis salam’s first son (Qabil),
because he was the first person to establish the way of (unjust) killing." (Sahih Bukhari, Al-I‘tisam, Chapter: The sin of one who calls to misguidance, Hadith: 7321; and Sahih Muslim, Al-Qasamah, Chapter: The sin of one who establishes killing, Hadith: 1677)

➍
This is also clarified by the context in which the Prophet sallallahu alayhi wa sallam said this:
The incident is that members of the tribe of Banu Mudar came to the Prophet sallallahu alayhi wa sallam in a pitiable state, not even having proper clothing.
On this occasion, the Prophet sallallahu alayhi wa sallam encouraged the noble Companions radi Allahu anhum to give charity. The first to respond was an Ansari Companion who brought such a heavy pouch filled with dirhams and dinars that it was slipping from his hand.
After that, so much charity was brought that on one side a heap of food and drink was formed, and on the other side a heap of clothes.
On this occasion, the Prophet sallallahu alayhi wa sallam made the aforementioned statement:
(Sahih Muslim, Zakat, Chapter: Encouragement to give charity even with half a date or a good word...etc., Hadith: 1016)

➎
Those engaged in da‘wah and tabligh (inviting and preaching) should act with caution so that, by seeing weak or fabricated ahadith, they do not begin to invite towards an act that is not established by an authentic hadith. Otherwise, not only will their effort be wasted, but they will also bear a great burden of sin.

➏
When someone is invited towards sin, Satan usually whispers that there is no harm in committing this sin; the sin will be upon the one who invited us to it. This is a great misunderstanding.
The one who commits the wrong act cannot escape its responsibility; however, the sin of the one who invites towards sin is more severe, so he will also be considered an equal partner in the crime of the perpetrator and will be deserving of punishment.

➐
This hadith contains great glad tidings for those who preach goodness.
For every person who adopts a good deed due to the effort of one individual, an equal amount of reward will automatically be recorded in his book of deeds.

➑
The reward and sin being recorded in the account of the inviter happens automatically; the intention or purpose of the one who acts upon it has no bearing on this. Therefore, it is not correct to use this hadith as evidence for the prevalent concept of "isal al-thawab" (conveying reward to the deceased), otherwise, just as isal al-thawab is accepted, the concept of "isal al-ithm" (conveying sin) would also have to be accepted.

Takhrij al-Hadith:
[صحيح مسلم 2351]

Fiqh al-Hadith:
➊ In this hadith, establishing a sunnah refers to a way that is proven from the Book and the Sunnah, but it should be remembered that this does not mean inventing an innovation (bid‘ah).
➋ Inviting people towards an act that is established from the Sunnah is a deed of great reward.
➌ Allah Ta‘ala sent the Messenger of Allah (sallallahu alayhi wa sallam) as a mercy to the worlds.
➍ If there is a severe need, it is permissible to appeal for cooperation before the people.
➎ If weapons are available, it is Sunnah to remain armed at all times.
➏ The only one who removes difficulties is Allah alone.
➐ Reciting verses (ayat) in the sermon (khutbah) to explain to the people is Sunnah.
➑ To become distressed upon seeing a troubled Muslim and to feel happiness at his relief is the essence of faith (iman).

Benefit:
It is understood that when any collective good deed is about to begin, one should strive to be the first to start it, because by doing so a person becomes deserving of a great reward without hardship.