Shaikh Abdul Muhsin Al-Abbaad (may Allaah preserve him) said:

The Messenger (sallal-laahu-alayhi-wasallam) said:

مَنْ أَحْدَثَ فِي أَمْرِنَا هَذَا مَا لَيْسَ مِنْهُ فَهُوَ رَدٌ

He who innovates something in this matter of ours [i.e., Islam] that is not from it will have it rejected [by Allah].” [Saheeh Al-Bukhaari. Number 2697] [Saheeh Muslim. Number 1718]

And in another wording of this hadeeth in Saheeh Muslim the Prophet said:

[مَنْ عَمِلَ عَمَلًا لَيْسَ عَلَيْهِ أَمْرُنَا فَهُوَ رَد ]

”He who does an act (of worship) which we have not commanded (i.e. not commanded by the Prophet), will have it rejected.”

And he (alayhis salaatu was-salaam) said:

 فَإِنَّهُ مَنْ يَعِشْ مِنْكُمْ فَسَيَرَى اخْتِلَافًا كَثِيرًا، فَعَلَيْكُمْ بِسُنَّتِي وَسُنَّةِ الْخُلَفَاءِ الرَّاشِدِينَ الْمَهْدِيينَ، عَضُّوا عَلَيْهَا بِالنَّوَاجِذِ، وَإِيَّاكُمْ وَمُحْدَثَاتِ الْأُمُورِ؛ فَإِنَّ كُلَّ بِدْعَةٍ ضَلَالَةٌ

Verily he lives long among you will see a lot of differing, so you must keep to my Sunnah and to the Sunnah of the Khulafaa ar-Rashideen (the rightly guided caliphs). Adhere to it with your molar teeth (i.e. adhere to it firmly). And beware of newly invented matters [in the religion], for every newly invented matter [in the religion] is a bidah (innovation in the religion), and every bidah is misguidance.” [Abu Dawood. Number 4607] [Tirmidhee. Number 2676]

(Also) the Messenger (sallal-laahu-alayhi-wasallam) has clarified in the hadeeth about the seventy three sects amongst whom seventy-two sects will enter the hell fire and one sect will be saved.  He clarified that this saved sect are those who adhere to the path him and his noble companions adhered to. 

Imaam Maalik [rahimahullaah] said: “The latter part of this Ummah (i.e. those who will come after the Salaf) will not be rectified except by way of that which rectified it’s earlier part (i.e. the Salaf).’’ And he [rahimahullaah) said: ‘’Whoever innovates a bidah in Islaam and considers it to be something good, then indeed he has claimed that Muhammad betrayed the Risaalah (i.e. the message he was entrusted with); because Allaah said: ٱلۡيَوۡمَ أَكۡمَلۡتُ لَكُمۡ دِينَكُمۡ -”This day, I have perfected your religion for you.” Therefore, what was not part of the religion at that time (i.e. during the time of the Messenger) cannot be part of the religion today (i.e. after his departure from this world).” [Al-Itisaam of Imaam Shaatibee 1/28]

It is not enough for a person to say: “I am going to do this act of worship even though it was not (performed by) the Prophet because my intention is pure and good.” The evidence (against this statement) is that when it reached the Messenger (sallal-laahu-alayhi-wasallam) that a man amongst his noble companions slaughtered the Eed sacrifice before the Eed Salaah, he (sallal-laahu-alayhi-wasallam) said to him: شَاتُكَ شَاةُ لَحْمٍ– “Your (slaughtered sheep was just mutton (not a sacrifice).” [Saheeh Bukhaari. Number: 5556]; meaning: It is not considered Eed sacrifice because it was not carried out in conformity with my Sunnah. Therefore, what is in conformity with the Sunnah is that the Eed sacrifice is done after the Eed Salaah; but as for sacrificing before the Salaah, then indeed it is outside its specified time and will not be regarded as (Eed sacrifice).

Al-Haafidh said in Fat-hul Baaree (10/17): “Shaikh Abu Muhammad Bin Abu Hamzah said: In this hadeeth is (established the fact that) if an action (i.e. an act of worship) is done with a sincere intention, it is still not (considered) righteous unless it is in conformity with the legislated divine revelation.

Also that which will shed more light on this affair is when Abdullaah Ibn Mas’ood came to a people who had gathered in the Masjid, and each of them had a number of stones/pebbles; and there was one man amongst them saying: ‘Say ‘Subhaanallaah’ a hundred times; say ‘Laa-ilaaha Ilal laah’ a hundred times; say ‘Allaahu Akbar’ a hundred times. So they repeated.

Then Abdullaah Ibn Mas’ood approached them and said: “What is this that you are doing?” They said: “O Abu Abdurrahman! These are pebbles to count the number of times we say Allahu Akbar, Laa ilaaha Illal laah and Subhaanallaah.” He said: “Count your sins and I assure you that you are not going to lose anything of your hasanaat. O Ummah of Muhammad, how quick is your destruction! These are the Companions of your Prophet present in great numbers; these are his clothes not worn out yet, and his pots are not broken yet. I swear by the One (i.e. Allaah) in Whose Hands my soul is, either you are following a religion that is better than the Religion of Muhammad or you are opening a door of misguidance.” They said: “O Abu Abdurrahman! We swear by Allaaah that we intend nothing other than good.’’ He said: How many people want good but never reach it! [Sunan Ad-Daarimee 1/68-69] [Also recorded by Imaam Albaanee in As-silsilah As-Saheehah. Number 2005][

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[Source: أثر العبادات في حياة المسلم- page:4-11. Abridged and slightly paraphrased]