The scholar Mohammed ibn Saleh Al-Othaimeen - may Allah have mercy on him - said in his message (Creativity in the Perfection of the Shari’ah and the Danger of Innovation):
"Let it be known, O brothers, that compliance is only achieved when the action is in accordance with the Shari’ah in six matters:
The first: the cause. If a person worships Allah in a way that is not legislated, it is an innovation that is rejected. For example, some people commemorate the 27th night of Rajab with the excuse that it is the night of the Prophet's ascension, so they engage in night prayers. Although it is an act of worship, it becomes an innovation when linked to a cause that is not established in the Shari’ah. Adhering to worship in accordance with the Shari’ah in terms of the cause is important to distinguish between innovation and what is considered part of the Sunnah.
The second: the type. Worship must conform to the Shari’ah in its type. If a person worships Allah in a way that is not legislated, it is not acceptable. For example, if a man sacrifices a horse, it is not a valid sacrifice because it contradicts the Shari’ah regarding the type of permissible sacrifices.
The third: the amount. If a person wants to increase the number of prayers beyond what is obligatory, it is considered an unacceptable innovation because it violates the prescribed amount in the Shari’ah. For example, if a person performs five rak’ah for the Dhuhr prayer, it is not valid according to consensus.
The fourth: the manner. If a man performs ablution by washing his feet first, then wiping his head, then washing his hands, and then his face, his ablution is invalid because it contradicts the prescribed manner in the Shari’ah.
The fifth: the time. If a man sacrifices an animal on the first day of Dhul-Hijjah, the sacrifice is not accepted because it violates the prescribed time. I have heard of some people who sacrifice sheep in Ramadan as an act of worship, but this is an innovation because there is nothing in Ramadan that warrants sacrificing for the purpose of drawing near to Allah, except for the prescribed Udhiyah, or Aqiqah. Sacrificing in Ramadan with the belief that the reward is similar to that of Eid al-Adha is considered an innovation. However, if the sacrifice is for the purpose of obtaining meat, then it is permissible.
The sixth: the place. If a man observes the seclusion (i'tikaf) in a place other than a mosque, his seclusion is not valid because seclusion can only be done in mosques. Similarly, if a woman says she wants to observe seclusion in the prayer area of her home, her seclusion is not valid due to contravening the Shari’ah regarding the place. For example, if a man wants to complete the Tawaf (circumambulation) around the Kaaba but finds the area restricted and begins to perform the Tawaf behind the mosque, his Tawaf is not valid because the designated location for Tawaf is the Kaaba, as stated by Allah to Ibrahim: "And [mention] when We made the House a place of return for the people and [a place of] security." (Quran, 2:125).".