What is the Judgment on Innovation in General:
Ahlussunnah agreed that some innovations are lawful (good) and some of them are unlawful (bad). They agreed that not every innovation is unlawful.
Allah praised the good innovation in surat alHadeed 27:
ثُمَّ قَفَّيْنَا عَلَى آثَارِهِمْ بِرُسُلِنَا وَقَفَّيْنَا بِعِيسَى ابْنِ مَرْيَمَ وَآتَيْنَاهُ الْإِنْجِيلَ وَجَعَلْنَا فِي قُلُوبِ الَّذِينَ اتَّبَعُوهُ رَأْفَةً وَرَحْمَةً وَرَهْبَانِيَّةً ابْتَدَعُوهَا مَا كَتَبْنَاهَا عَلَيْهِمْ إِلَّا ابْتِغَاءَ رِضْوَانِ اللَّهِ فَمَا رَعَوْهَا حَقَّ رِعَايَتِهَا فَآتَيْنَا الَّذِينَ آمَنُوا مِنْهُمْ أَجْرَهُمْ وَكَثِيرٌ مِنْهُمْ فَاسِقُونَ
[سورة الحديد/ءاية ٢٧]
Qur’an praised the Muslim followers of Prophet `Isa who had innovated Rahbaneeyyah, which Allah did not make it incumbent on them. Nevertheless, they did it to acquire Allah’s blessings. Rahbaneeyyah is to abstain from fulfilling one’s desires including marriage. Those followers built simple houses from clay away from the people and devoted themselves to worshipping Allah.
The Prophet distinguished between the good and the bad innovation. He said, as narrated by Muslim:
من سنّ في الإسلام سنّةً حسنة فله أجرها وأجر من عمل بها بعده من غير أن ينقص من أجورهم شىء، ومن سنّ في الإسلام سنّة سيئة كان عليه وزرها ووزر من عمل بها من بعده من غير أن ينقص من أوزارهم شىء.
It means that the one who innovates a good innovation acquires its reward and the reward similar to the rewards of those who follow him, but their rewards are not diminished. And the one who innovates a bad innovation acquires its sin and the sin similar to the sins of those who follow him, but their sins are not diminished.
AlBukhariyy and Muslim narrated that the Prophet said:
منْ أحْدثَ في أمرِنا هذا ما ليسَ منه فهو رَدٌّ
It means that if anyone innovates in our Religion a matter which does not belong to it (ما ليس منه), then it is rejected. Because the Prophet specifically pointed to the matters that do not conform to the rules of our Religion, it follows that not any innovation in Islam is rejected. Hence, the innovation which complies with the Qur’an, Hadeeth, Consensus and sayings and practices of the Companions is not rejected (it belongs to the innovations of guidance).
Imam ashShafi`iyy said: the innovation is of two kinds; the innovation of guidance, and the innovation of misguidance. He said: what complies with the Qur’an, Hadeeth, Consensus, and sayings and practices of the Companions is an innovation of guidance, and that which contradicts the Qur’an, Hadeeth, Consensus, or sayings and practices of the Companions is an innovation of misguidance. Imam al-Bayhaqiyy, `Izz bin `Abd-asSalaam, anNawawiyy, and Ibn Hajar al`Asqalaniyy gave the same definition of the two types of innovations. The four schools agreed on that classification.