𝑰𝒃𝒏 𝑻𝒂𝒚𝒎𝒊𝒚𝒚𝒂𝒉 𝒄𝒂𝒔𝒖𝒂𝒍𝒍𝒚 𝒓𝒆𝒇𝒖𝒕𝒆𝒔 “𝑩𝒊𝒅’𝒂𝒉 𝑯𝒂𝒔𝒂𝒏𝒂𝒉”. 𝑯𝒆 𝒓𝒆𝒇𝒖𝒕𝒆𝒅 𝒕𝒉𝒆𝒊𝒓 𝒔𝒖𝒑𝒑𝒐𝒔𝒆𝒅 𝒆𝒗𝒊𝒅𝒆𝒏𝒄𝒆𝒔 𝒃𝒚 𝒉𝒊𝒈𝒉𝒍𝒊𝒈𝒉𝒕𝒊𝒏𝒈 𝒕𝒉𝒆 𝒍𝒊𝒏𝒈𝒖𝒊𝒔𝒕𝒊𝒄 𝒎𝒆𝒂𝒏𝒊𝒏𝒈 𝒐𝒇 𝑩𝒊𝒅’𝒂𝒉, 𝒉𝒆 𝒕𝒉𝒆𝒏 𝒑𝒓𝒐𝒗𝒊𝒅𝒆𝒅 𝒄𝒍𝒆𝒂𝒓 𝒑𝒓𝒐𝒐𝒇𝒔 𝒇𝒓𝒐𝒎 𝒕𝒉𝒆 𝒔𝒑𝒆𝒆𝒄𝒉 𝒐𝒇 𝒕𝒉𝒆 𝒑𝒓𝒐𝒑𝒉𝒆𝒕 ﷺ 𝒂𝒏𝒅 𝒕𝒉𝒆 𝒂𝒄𝒕𝒊𝒐𝒏𝒔 𝒐𝒇 𝒕𝒉𝒆 𝑺𝒂𝒉𝒂𝒃𝒂𝒉 𝒕𝒐 𝒔𝒉𝒐𝒘 𝒕𝒉𝒂𝒕 𝒊𝒏𝒅𝒆𝒆𝒅 𝒆𝒗𝒆𝒓𝒚 𝑩𝒊𝒅’𝒂𝒉 𝒊𝒔 𝒎𝒊𝒔𝒈𝒖𝒊𝒅𝒂𝒏𝒄𝒆.

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It has been established in the Sahih (authentic collections) that our Prophet ﷺ used to say in his sermon: "The best of speech is the Book of Allah, the best of guidance is the guidance of Muhammad, and the worst of matters are the newly-invented ones, and every innovation (Bid’ah) is a misguidance."
What is meant by Bid'ah is: that for which no legal (Shari’i) evidence has been established to show that it is obligatory or recommended, whether it was performed during his ﷺ time or not. An example is the expulsion of Jews and Christians from the Arabian Peninsula or fighting the Turks; since these were done by his command, they were not innovations even if they were not executed during his lifetime.
Similarly, the collection of the Qur’an into a single volume (Mushaf), the gathering of people for the night prayers (Qiyam) in Ramadan, and similar actions are those whose obligation or recommendation has been established by legal evidence.
As for the statement of Umar (may Allah be pleased with him) regarding the Tarawih prayer: "What an excellent innovation this is," he meant innovation in the linguistic sense, because linguistically, an innovation is anything done without a prior example. This is similar to the words of Allah the Exalted: "Say: I am not a new thing (Bid’an) among the messengers" [Al-Ahqaf: 9]. It is not a religious innovation (Bid'ah fi al-Shari’ah), for every religious innovation is misguidance, as the Prophet ﷺ informed us.
Among the scholars, those who say that innovation is divided into "good" and "not good" (bad) are referring back to the linguistic classification.
Whenever it is said: "Every innovation is misguidance," the meaning is the religious innovation. Do you not see that the scholars among the Companions and the Successors (Tabi'un) who followed them in righteousness rejected the Adhan (call to prayer) for prayers other than the five daily prayers, such as the two Eid prayers, even though there was no specific prohibition against it? Similarly, they rejected praying after the Sa’i between Safa and Marwa based on an analogy with the prayer after Tawaf, and things of that nature.

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What the Messenger ﷺ left out (did not do), despite the motive for doing it being present, then leaving it is a Sunnah and doing it is a blameworthy innovation. This means that if the full motive was present during his life—just as it was after his death—what he left was a Sunnah and doing it is an innovation.
This is in contrast to what he left because the motive was absent, and the motive only appeared after his death, such as collecting the Qur’an into a Mushaf or expelling the Jews and Christians from the Arabian Peninsula.
It is also in contrast to what he left because of a barrier (preventative factor), such as gathering for the Tarawih prayer; the full motive includes the absence of the barrier.
In the Sunan and Al-Tirmidhi, there is a hadith which Al-Tirmidhi authenticated, the hadith of Irbad bin Sariyah (may Allah be pleased with him) who said: "The Messenger of Allah ﷺ gave us an eloquent exhortation that caused eyes to tear and hearts to tremble. Someone said: 'O Messenger of Allah, it is as if this were a farewell exhortation, so what do you enjoin upon us?' He said: 'I enjoin you to have Taqwa (piety) of Allah, and to hear and obey... for indeed, those of you who live after me will see much conflict. So, you must follow my Sunnah and the Sunnah of the Rightly Guided Caliphs after me. Hold fast to it and bite onto it with your molars. Beware of newly-invented matters, for every newly-invented matter is an innovation, and every innovation is misguidance.'" In one version: "And every misguidance is in the Fire."
The root of a corrupt religion is either worshipping other than Allah, the Mighty and Majestic, or worshipping Him in a way that He did not permit—either by prohibiting what Allah has not prohibited, or by permitting what Allah has prohibited.
“ᴍᴀʏ ᴀʟʟᴀʜ ʀᴇᴡᴀʀᴅ ᴛʜᴇ ꜱᴄʜᴏʟᴀʀꜱ ᴏꜰ ᴀʜʟᴜꜱ ꜱᴜɴɴᴀʜ ᴡʜᴏ ᴄʟᴀʀɪꜰɪᴇᴅ ᴛʜᴇ ᴛʀᴜᴛʜ ᴀɴᴅ ᴇxᴘᴏꜱᴇᴅ ꜰᴀʟꜱᴇʜᴏᴏᴅ, ᴀɴᴅ ᴍᴀʏ ʜᴇ ᴍᴀᴋᴇ ᴜꜱ ᴀᴍᴏɴɢ ᴛʜᴏꜱᴇ ᴡʜᴏ ʟɪꜱᴛᴇɴ, ᴜɴᴅᴇʀꜱᴛᴀɴᴅ, ᴀɴᴅ ꜰᴏʟʟᴏᴡ.”