Hadith: Whosoever introduces a good practice in Islam, there is for him its reward and the reward of those who act upon it after him without anything being diminished from their rewards. And whosoever introduces an evil practice in Islam will shoulder its sin and the sins of all those who will act upon it, without diminishing in any way their burden

عن جرير بن عبد الله رضي الله عنه قال: كُنَّا في صَدرِ النَّهَار عِند رسول الله صلى الله عليه وسلم فَجَاءه قَومٌ عُرَاةٌ مُجْتَابِي النِّمَار أَو العَبَاءِ، مُتَقَلِّدِي السُّيُوف، عَامَّتُهُم مِن مُضَر بَل كُلُّهُم مِن مُضَر، فَتَمَعَّر رسول الله صلى الله عليه وسلم لِمَا رَأَى بِهِم مِنَ الفَاقَة، فدَخَل ثُمَّ خَرج، فأَمَر بِلاَلاً فَأَذَّن وَأَقَام، فَصَلَّى ثُمَّ خَطَب، فقال: «(يا أيها الناس اتقوا ربكم الذي خلقكم من نفس واحدة) إلى آخر (إن الله كان عليكم رقيبًا) ، والآية الأخرى التي في آخر الحشر: (يا أيها الذين آمنوا اتقوا الله ولتنظر نفس ما قدمت لغد) تَصَدَّق رَجُلٌ مِن دِينَارِهِ، مِن دِرهَمِهِ، مِن ثَوبِهِ، مِن صَاعِ بُرِّهِ، مِنْ صَاعِ تَمرِهِ -حتَّى قال- وَلَو بِشِقِّ تَمرَة» فَجَاء رَجُلٌ مِنَ الأَنصَار بِصُرَّةٍ كَادَت كَفُّهُ تَعجَزُ عَنْهَا، بل قَد عَجَزَت، ثُمَّ تَتَابَع النَّاسُ حَتَّى رَأَيتُ كَومَين مِن طَعَامٍ وَثِيَابٍ، حَتَّى رَأَيتُ وَجهَ رَسُول الله صلى الله عليه وسلم يَتَهَلَّلُ كَأَنَّه مُذْهَبَة.ٌ فقال رسول الله صلى الله عليه وسلم : «مَنْ سَنَّ فِي الإِسلاَم سُنَّةً حَسَنَةً فَلَهُ أَجْرُهَا، وَأَجْرُ مَنْ عَمِلَ بِهَا بَعْدَهُ، مِنْ غَيرِ أَنْ يَنْقُصَ مِن أُجُورِهِم شَيءٌ، وَمَنْ سَنَّ فِي الإِسلاَم سُنَّةً سَيِّئَةً كَانَ عَلَيه وِزْرُهَا، وَوِزرُ مَنْ عَمِلَ بِهَا مِنْ بعدِه، من غير أن ينقُص مِن أَوزَارِهَم شيء».
[صحيح] - [رواه مسلم]

Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported: We were with Messenger of Allah (may Allah's peace and blessings be upon him) shortly after dawn when there came to him some people clad in woolen rags, or covered with sleeveless blankets; and with swords hanging down from their necks. Most of them, rather all of them, belonged to the Mudar tribe. The Prophet's face changed when he saw them starving. Then he went into his house and came out; then he commanded Bilāl to proclaim the Adhān. So he proclaimed the Adhān and recited Iqāmah and the Prophet led the prayer. Then he delivered a sermon saying: (O mankind, fear your Lord, who created you from one soul and created from it its mate and dispersed from both of them many men and women. And fear Allah, through whom you ask one another, and the wombs. Indeed Allah is ever, over you, an Observer) [Sūrat An-Nisā':1]. He also recited a verse at the end of Sūrat al-Hashr: (O you who believe, fear Allah and keep your duty to Him. And let every one look what he has sent forth for the tomorrow.) Thereafter, every man gave in charity dinar, dirham, clothes, measure-fulls of wheat and measure-fulls of dates till he said: "(Give in charity) be it half a date." Then a man from the Ansār came with a bag which was difficult for him to hold in his hand. Thereafter, the people came successively (with charity) till I saw two heaps of food and clothes. I noticed that the Messenger's face was glowing like that of the bright moon or glittering gold. Then he said: "Whosoever introduces a good practice in Islam, there is for him its reward and the reward of those who act upon it after him without anything being diminished from their rewards. And whosoever introduces an evil practice in Islam will shoulder its sin and the sins of all those who will act upon it, without diminishing in any way their burden."
[Authentic hadith] - [Narrated by Muslim]

Explanation

This Hadīth shows the Prophet's care and compassion towards his followers. As the Companions were with the Prophet, early in the day, there came a group of people, mostly or entirely from Mudar tribe, wearing very simple clothes, which indicated their poverty, and hanging their swords, a sign of readiness to receive order for Jihād. The Prophet's face changed when he saw their poor state despite belonging to Mudar, one of the noblest Arab tribes. So, he entered home and then went out. He ordered Bilāl to proclaim the Adhān and then he offered the prayer and addressed the people. Praising Allah, the Almighty, as usual, he recited the verses that reads: (O mankind, fear your Lord, who created you from one soul and created from it its mate and dispersed from both of them many men and women. And fear Allah, through whom you ask one another, and the wombs. Indeed Allah is ever, over you, an Observer) [Sūrat An-Nisā':1]; (O you who believe, fear Allah and keep your duty to Him. And let every one look what he has sent forth for the tomorrow. Indeed, Allah is Acquainted with what you do) [Sūrat al-Hashr: 18]. Then, he encouraged the people to give charity, and in response they began to give, and a man would give his dinar in charity, another would give a dirham, and others would give their garment or a Sā‘ of wheat or dates. Then, he encouraged charity even if it were half a date. The Companions (may Allah be pleased with them) were most keen on doing good, rushing and vying towards this. One of them came with a bundle that was almost too heavy for him to carry and placed it in front of the Prophet. Seeing all this goodness and generosity, which would meet the need of those poor people, the Prophet (may Allah's peace and blessings be upon him) was very pleased, and he said: "Whosoever introduces a good practice in Islam, there is for him its reward and the reward of those who act upon it after him without anything being diminished from their rewards. And whosoever introduces an evil practice in Islam will shoulder its sin and the sins of all those who will act upon it, without diminishing in any way their burden." "A good practice" here refers to something already established and rooted in the religion, not contrived and added to it.