The Holy Prophet has given the same definition in the following long Hadith: -
Hazrat Irmad bin Sariah reports that, “Once the Holy Prophet led us in the Morning Prayer. Then turning towards us he gave a sermon that was so pithy that tears began to flow from the eyes of the audience and hearts began to tremble with fear. A Companion said: "O Apostle of Allah! This sermon seems to be such as if it is the last and the farewell sermon. So give us some advice (i.e., some consummate thing that we may grasp firmly). At this the Holy Prophet said: 'I advise you to adopt obedience to Allah and to adopt obedience to the amir even though he be a Negro slave. He who survives me will see great dissensions. So keep grasping mine and the well-guided successors' method firmly; follow it and grasp it with your teeth, and keep abstaining from all new things for every new thing in religion is an innovation and every innovation is deviation."
Hazrat Ali says that, by Allah, the Sunnah is the Holy Prophet's method, and innovation is that which may be repugnant to the Sunnah. By this it is known that to invent things as religion other than the Holy Prophet's methods and the shari'ah brought by him is innovation.
The comminations that we have quoted earlier from the Quran and the Hadith concerning opposition to Allah and the Holy Prophet and the Sunnah are applicable here also. I, therefore, quote here those verses and hadiths that I have not quoted earlier; however, at some places, due to some expediency, repetition is also possible. Allah Most Glorious says: -
"And whoso opposeth the messenger after the guidance (of Allah) hath been manifested unto him, and followeth other than the believer's way. We appoint for him that unto which he himself hath turned, and expose him unto hell - a hapless journey’s end." (IV: 115)
Similarly, it is said at another place: -
"Lo! Those who disbelieve and turn from the way of Allah and oppose the messenger after the guidance hath been manifested unto them, they hurt Allah not a jot, and He will make their actions fruitless." (XLVII: 32).
Thus the people whom Satan involves in innovation, throughout their lives they recede from the Shari'ah and undertake wrong strivings and waste their labour; consequently they get nothing but loss and the deeds go to waste.