Innovation is that which opposes the Sunnah and it consists of two types.
Firstly: Innovations with regards to beliefs, and it is to have a belief which is in opposition to that which Allāh and His Messenger (ﷺ) have informed us of. This is mentioned in his (ﷺ) statement,
And my Ummah will divide into 73 sects, all in the fire except one. They (the companions) asked, ‘and who are they O Messenger of Allāh?’ He (ﷺ) replied, they are those who are upon that which I and my companions are upon today.
So whoever resembles this description (mentioned by the Messenger (ﷺ)) is truly a person of Sunnah, and whosoever is from the remaining sects, then he is an innovator—every innovation is misguidance—and innovations vary according to their straying from the Sunnah.
Secondly: Innovations with regards to actions, and it is to worship (Allāh) with other than that which He and His Messenger (ﷺ) have legislated, or prohibiting that which Allāh or His Messenger (ﷺ) made permissible. Whosoever worships (Allāh) with other than that which was legislated or prohibits that which the Legislator (Allāh) has not prohibited is an innovator.1
1. Reported by Imām Aḥmad in his Musnad 2/332, Abū Dāwūd in his Sunan 5/4, Tirmidhī in his Sunan, Kitāb al-Īmān 5/25, Ibn Mājah in his Sunan, Kitāb al-Fitan 2/1321.
Source: Suʾāl wa Jawāb fī Ahim al-Muhammāt by the noble scholar al-ʿAllāmah ʿAbd al-Raḥmān ibn Nāṣir al-Sʿadī, Question #17, pgs. 22-23
Translated by: Munīb al-Ṣumālī