Shaykh Abd Allaah bin Muhammad bin Abd al-Wahhaab writes (al-Durar al-Saniyyah 1/237):

هذا ومما نحن عليه : أن البدعة، وهي : ما حدثت بعد القرون الثلاثة، مذمومة مطلقاً، خلافاً لمن قال حسنة، وقبيحة ؛ ولمن قسمها خمسة أقسام، إلا إن أمكن الجمع، بأن يقال : الحسنة ما عليه السلف الصالح، شاملة : للواجبة، والمندوبة، والمباحة ؛ ويكون تسميتها بدعة مجازاً ؛ والقبيحة ما عدى ذلك، شاملة : للمحرمة، والمكروهة ؛ فلا بأس بهذا الجمع

And from that which we are upon: That bid'ah (innovation) is that which arose after the three generations, it is blameworthy absolutely (without qualification), in opposition to the one who said "hasanah" (good) and "qabeehah" (repugnant), and likewise (in opposition) to the one who divided it into five categories. Except that it is possible to reconcile (between the two ways) in that it can be said: That which is "hasanah" (good" is what the Righteous Salaf were upon, comprising of that which is (classified as) waajibah (obligatory), mandubah (recommended), and mubaahah (permissible), and its being named "bid'ah" is figurative. And the "qabeehah" (repugnant) is whatever is besides that, comprising the muharramah (unlawful) and makruhah (disliked). So there is nothing wrong in this reconciliation.

And also, there occurs (al-Durar al-Saniyyah 1/237):

وأما مذهبنا : فمذهب الإمام أحمد بن حنبل، إمام أهل السنة، ولا ننكر على أهل المذاهب الأربعة إذا لم يخالف نص الكتاب والسنة، ولا إجماع الأمة ، ولا قول جمهورها ؛ والمقصود : بيان ما نحن عليه من الدين، وأنه عبادة الله وحده لا شريك له فيها، بخلع جميع الشرك، ومتابعة الرسول فيها، نخلع جميع البدع إلا بدعة لها أصل في الشرع، كجمع المصحف في كتاب واحد، وجمع عمر رضي الله عنه الصحابة . ( ص246) على التراويح جماعة، وجمع ابن مسعود أصحابه على القصص كل خميس، ونحو ذلك، فهذا حسن، والله أعلم

And as for our madhhab: Then the madhhab of Imaam Ahmad bin Hanbal, the Imaam of Ahl al-Sunnah, and we do not show rejection against the adherents to the four madhhabs when they do not oppose a text from the Book, the Sunnah or the consensus of the Ummah, or the saying of their majority. And the intent here is to explain what we are upon (in the matter) of deen, that it is worshipping Allaah alone, without any partners therein, whilst letting go of all (forms of) Shirk, and following the Messenger therein, and letting go of all (forms of) innovation except the innovation which has a basis in the Shari'ah, such as the gathering of the mus-haf in a single book, and Umar gathering the companions in the taraaweeh in congregation, and Ibn Mas'ood gathering his companions (students) to narrate (to them) on every Thursday and what is like this. This, is good (innovation), and Allaah knows best.

Notes

The classifications of bidah used by certain scholars and the use of the phrases such as "praiseworthy innovation" and "bidah hasanah" were never ever intended by these scholars to justify innovations in worship. Rather, they used these terms because they thought that when the Messenger (alayhis salaam) said (كل بدعة ضلالة), he was using the word bidah from a purely linguistic meaning. Upon this assumed understanding it is perfectly legitimate and appropriate to say that this statement must be qualified to exclude what is not otherwise blameworthy in the religion, which would include the following:

• Matters of broad public interest (maslahah mursalah) which may either be obligatory or recommended, hence, the examples given by al-Izz bin Abd al-Salam and followed by al-Nawawi, such as being concerned with Arabic grammar (obligatory) and building schools (recommended) and authoring works to refute innovators who were clearly not present in the time of the Prophet (sallallaahu alayhi wasallam).

• Matters which are merely permitted, which include enjoyment of new forms and types of food, drink, clothing, residence. Hence, bid'ah (innovation) in relation to these matters, takes on the rulings of those matters, it is permissible.

• Matters of goodness which in and of themselves are established in the Shariah (giving charity for example), but which a person enacts from his own initiative (in a way no one did before him) and is subsequently followed by others. An example of this could be when a person takes half his savings and travels to poverty-stricken area and feeds the people and builds wells for them. So he is then followed by a dozen others. So no new act has been innovated here into Islam, but a person has simply performed an act which no one preceded him in, and it became an example for others besides him. So he instituted a "sunnah hasanah" to use the Shariah term, or if you want to use the language of others, "bidah hasanah", in other words, an innovation purely with its linguistic (not Shariah) meaning, and this is true because prior to this man, no one else took half of his own wealth and travelled to the poverty stricken place and fed the people and built wells for them.

This was the intent of the Shafi'ite jurists, operating upon the assumption that the Messenger (sallallaahu alayhi wasallam) used the word bid'ah from a linguistic perspective, they wanted to ensure - through their classification and subsequent elaboration - that the abovementioned matters are not wrongly classified as the bid'ah that is blameworthy and condemned in the religion. Whilst this seems perfectly fine, these jurists departed from the understanding of the majority of the Scholars that the Messenger (sallallaahu alayhi wasallam) was using the word bid'ah in its legislative sense only, and thus, the classification of bid'ah into praiseworthy and blameworthy is unnecessary. For a detailed elaboration of this matter refer to the following in-depth series of articles:

• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 8 - Imam Al-Nawawi's Refutation of Contemporary Innovators Trying to Justify bidah hasanah in Matters of Worship - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 7 - A Diagrammatical Representation of the True and Real Intent of the Shafi'ite Jurists and Unveiling the Deception of the Contemporary Innovators - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 6 - Al-Izz Bin Abd Al-Salam's Refutation of a Claimed 'Bidah Hasanah' and Refutation of the Doubts of Contemporary Innovators - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 5 - al-Izz bin Abd al-Salam on Adherence to the Sunnah and Avoiding Innovation in Worship - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 4 - Al-Izz Bin Abd Al-Salam's Classification of Bid'ah - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 3 - the Statements of Imaam al-Shafi'i - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 2 - Understanding Matters of public interest (al-masalih al-mursalah) - (see here)
• Refuting the Notion of bid'ah hasanah (good innovation) in Worship: Part 1 - Ibn Hajar Al-Asqalani on the Usage of the Word Bid'ah in Its Linguistic Sense - (see here)