We know that some religious innovations contradict the core of faith, and some contradict the perfect state of faith. Religious innovations that comprise polytheism or disbelief contradict the core of faith; whereas religious innovations that have no polytheism or disbelief do contradict its perfect state.
We believe that Allah Almighty has perfected the religion for us and completed His favor upon us. He, Exalted be He, says: {This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion.} [Surat al-Mā’idah: 3] And the Prophet (may Allah’s peace and blessings be upon him) said: "Whoever introduces something to this matter of ours (religion) which is not part of it, will have it rejected." [Narrated by Al-Bukhāri (2697), Muslim (1718), Abu Dāwūd (4606), and Ibn Mājah (14)]
We know that religious innovations are among the things that contradict the perfect state of faith. The Prophet (may Allah’s peace and blessings be upon him) said: "Beware of newly introduced things (to religion), for every newly introduced thing is a religious innovation, and every religious innovation is a misguidance." [Narrated by Abu Dāwūd (4607), At-Tirmidhi (2676), Ibn Mājah (42), Ahmad (17144), Ibn Abi ‘Āsim in As-Sunnah (26), and Ibn Waddah in Al-Bida‘ (54)]
Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "Whoever calls to guidance will obtain a reward equivalent to that of those who follow him, without their reward being diminished in the least; and whoever calls to misguidance will incur a sin equivalent to that of those who follow him, without their sins being diminished in the least." [Narrated by Muslim (2674), Abu Dāwūd (4609), At-Tirmidhi (2674), and Ibn Mājah (206)]
Al-Bukhāri (may Allah have mercy upon him) set a chapter in his Sahīh, titled: "Chapter on the sin of he who calls to misguidance or introduces a bad practice". He cited therein the Hadīth that reads: "No soul gets killed wrongfully without the first son of Adam taking a share of the guilt for its blood because he was the first to commit murder." [Narrated by Al-Bukhāri (7321), Muslim (1677), At-Tirmidhi (2673), An-Nasā’i (3985), and Ibn Mājah (2616)]
Abu Sa‘īd (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "You will follow the ways of those who were before you, span by span and cubit by cubit, so much so that even if they were to enter a hole of a mastigure, you would follow them." We said: "O Messenger of Allah, the Jews and Christians?" He said: "Who else then?" [Narrated by Al-Bukhāri (3456) and Muslim (2669)]
We know that Allah Almighty forbade dissension and disunity. He, Blessed and Glorified be He, says: {Do not be like those who became divided and differed after the clear proofs had come to them; and for them there will be a great punishment.} [Surat Āl ‘Imrān: 105] He, Exalted be He, also says: {So let those who disobey his command beware lest some trial may afflict them or they may be afflicted with a painful punishment.} [Surat an-Nūr: 63]
We know that religious innovations include extolment of graves and constructing buildings over them. This may also lead to Shirk. Another religious innovation is making images of the righteous people in the hope of following their example after they pass away. The Prophet (may Allah’s peace and blessings be upon him) said: "Indeed, those who were before you used to take the graves of their prophets and righteous men as places of worship. Indeed, you should not take graves as places of worship. I forbid you to do that." [Narrated by Muslim (532)]
‘Ā’ishah (may Allah be pleased with her) reported: Umm Habībah and Umm Salamah mentioned a church they had seen in Ethiopia, and there were pictures in that church. When they told the Prophet (may Allah’s peace and blessings be upon him) about that, he said: "Those people are such that if a pious man amongst them died, they would build a place of worship over his grave and paint those pictures in it. Those people will be Allah's worst creatures on the Day of Resurrection." [Narrated by Al-Bukhāri (427), Muslim (528), and An-Nasā’i (704)]
The Prophet (may Allah's peace and blessings be upon him) ordered Muslims to obliterate images and level graves. Abu al-Hayyāj al-Asdi related that ‘Ali said to him: "I am sending you on the same mission as the Messenger of Allah (may Allah's peace and blessings be upon him) sent me. Do not leave an image without obliterating it, or a high grave without levelling it." [Narrated by Muslim (969), Abu Dāwūd (3218), At-Tirmidhi (1049), and An-Nasā’i (2031)]
We know that wrong innovations include such newly introduced festivities and joining the disbelievers in their festive occasions. Anas ibn Mālik (may Allah be pleased with him) reported: When the Messenger of Allah (may Allah's peace and blessings be upon him) came to Madīnah, the people had two days on which they engaged in games. He asked: "What are these two days?" They said: "We used to enjoy ourselves on them in jāhiliyyah." The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Indeed, Allah has substituted for you something better than them: the Day of Al-Ad'ha and the Day of Al-Fitr." [Narrated by Abu Dāwūd (1134), An-Nasā'i (1556), Isma‘īl ibn Ja‘far, in the Hadīth by ‘Ali ibn Hajar (62), Ahmad (12827), and ‘Abd ibn Humayd (1392)]
Allah, the Exalted, says: {For every community We ordained ways of worship which they follow, so do not let them dispute with you [O Prophet] in this matter. But call them to your Lord, for you are indeed on the straight guidance.} [Surat al-Hajj: 67] Commenting on the words {For every community We ordained ways of worship which they follow}, Ibn ‘Abbās (may Allah be pleased with him) said: "An Eid." [Tafsīr At-Tabari (18/679) and narrated by Ibn Abi Hātim in At-Tafsīr (14718)] And the Prophet (may Allah's peace and blessings be upon him) said: "Every people have their Eid, and this is our Eid." [Narrated by Al-Bukhāri (952), Muslim (892), Abu Dāwūd (1593), and Ibn Mājah (1898)]
We know that one of the wrong innovations is seeking Barakah (blessing) in something Allah Almighty did not make a cause of blessing. This may also lead to Shirk. Abu Wāqid al-Laythi (may Allah be pleased with him) reported: When the Messenger of Allah (may Allah's peace and blessings be upon him) went out to Hunayn, he passed by a tree that the idolaters called Dhāt Anwāt upon which they hung their weapons. The Companions said: "O Messenger of Allah, make a Dhāt Anwāt for us as they have a Dhāt Anwāt." The Prophet (may Allah's peace and blessings be upon him) said: "Glory be to Allah! This is like what Mūsa's people said: {Make for us a god just as they have gods} [Surat al-A‘rāf: 138]. By the One in Whose Hand my soul is, you will follow the way of those who were before you." [Narrated by At-Tirmidhi (2180), Mu‘ammar in Al-Jāmi‘ (20763), ‘Abdur-Razzāq in At-Tafsīr (931), and Ibn Abi Shaybah (38530)]