According to Muslim scholars, bid'ah in worship is divided into two categories, namely good innovation and sinful innovation. Among the scholars that divide it into the two categories are:
1. Imam Shafi'i
According to Imam Shafi'i, the bidah is divided into two types, namely praiseworthy innovation (bid'ah mahmudah) and strongly criticized innovation (bid'ah madzmumah). So any innovation which suits the sunna is praiseworthy (mahmudah), and that does no suit the sunna is strongly criticized (madzmumah).
The good/praiseworthy innovation is divided into two parts. The first is obligatory innovation, such as codification (collection) of the Qur'an at the time of Caliph Utsman ibn Affan and the collection of hadiths into main books in the days afterward. While the second is permissible bid'ah, such as praying 20 cycles of Tarawih in the time of Caliph Umar ibn Khattab.
According to Imam Bayhaqi, the bid'ah is divided into two parts; criticized innovation (bid'ah madzmumah) and praiseworthy innovation (bid'ah ghairu madzmumah). Any innovation not violating the Qu'ran, the Sunna, and (ulema) consensus is praiseworthy (mahmudah) or (ghairu madzmumah). While reprehensible innovation (madzmumah) is that having no religious grounds (syar'i) at all.
3. Imam Nawawi
According to Imam Nawawi, the bid'ah is divided into two parts, namely praiseworthy innovation (bid'ah hasanah) and sinful innovation (bid'ah qabihah).
4. Imam Hafidz Ibn al-Atsir
Ibn Atsir also divides the bid'ah into two parts, namely the bid'ah having religious grounds from the Qur'an and the Hadith, and the one having no religious grounds from the two.
So any form of innovation that violates the Qur'an and the Hadith/Sunna is reprehensible and must be denied. Instead, any form of innovation that generally suits religious arguments could be considered praiseworthy.
And what about the hadith:
كل بدعة ضلالة، وكل ضلالة في النار
Every innovation is a misguidance.
Here is the opinion of some Muslim scholars:
1. Imam Nawawi
The above hadith is included in the category of 'am (general) that should be specialized / itemized (takhshish).
2. Imam al-Hafidz Ibn Rajab
The above hadith is in the category of 'am but the desired is khash ('am yuridu bihil khash). This means the hadith is textually general in nature, but in its meaning the hadith requires details.
There are some scholars dividing the bid'ah into five types.
1. The bid'ah that must be done: for example studying nahwu (Arabic grammar), studying systematic theology arguments aimed to demonstrate (the truth) to atheists and those who disbelieve in Islam, etc.
2. The bid'ah that is recommended (mandub): for example calling for prayer using loudspeakers, printing scientific books, building Islamic schools, and so forth.
3. The bid'ah that is permissible: for example, creating a colorful dish of food, and the like.
4. The bid'ah that is objectionable (makruh): for example, being excessive in decorating Manuscripts, mosques and so forth.
5. The bid'ah that is forbidden (haram). It is any new thing in terms of religion that is considerably opposed to the syar'i propositions in general. such as doing seven cycles of evening prayers (isya'), etc.
(excerpted from the book "NU Traditions and Their Arguments, published by LTM (Institute for Mosque Management) of the Central Board of Nahdlatul Ulama / PBNU)