Shaykh Saalih al-Fawzaan (may Allaah preserve him) said:
This label is wrong with regard to wording, and with regard to the meaning.
With regard to wording, that is because the da’wah is not being attributed to the one who started it, namely Shaykh Muhammad, rather it is being attributed to ‘Abd al-Wahhaab, who had nothing to do with it. The proper name for it would be al-Da’wah al-Muhammadiyyah (“the Muhammadan call”).
But these opponents realized that this would be a good name that would not put anyone off, so they changed it to this distorted name.
With regard to the meaning, that is because this da’wah did not drift away from the way of the righteous salaf, namely the Sahaabah, Taabi’een and their followers. So it should rightly be called the Salafi da’wah, because the one who founded it did not introduce any innovation in that which was named after him, as the promoters of misguided sects such as the Ismailis and Qarmatiyyah introduced innovations. If those misguided sects had called themselves salafis, the people and the historical record would have rejected this name, because they are outside the path of the salaf and were introduced by their founders.
The correct name, both in wording and meaning, for the call of Shaykh Muhammad ibn ‘Abd al-Wahhaab is the Muhammadan da’wah or the Salafi da’wah.
But because this name annoys the enemies, they used an incorrect name. Therefore the word Wahhaabi was not known among the followers of the Shaykh, and it is their opponents who gave them this offensive nickname. Indeed, they gave this name to everyone who follows the way of the salaf, even in India, Egypt, Africa and elsewhere. By giving it this label, its opponents wanted to isolate the da’wah from the right path. So they excluded it from the four madhhabs and counted it as a fifth madhhab, “out of envy from their ownselves, even after the truth has become manifest unto them” (al-Baqarah 2:109).
Source: al-Bayaan li Akhta’ Ba’d al-Kuttaab (70)