Linguistically bid'ah (innovation) means 'a newly invented matter'. The Sharee'ah definition of bid'ah is: "A newly invented way [beliefs or action] in the religion, in imitation of the Sharee'ah (prescribed Law), by which nearness to Allaah is sought, [but] not being supported by any authentic proof - neither in its foundations, nor in the manner in which it is performed." Al-I'tisaam of ash-Shaatibee (1/37).
The Messenger of Allaah (sallallaahu alaihi wasallam) said: "Every innovation is misguidance and going astray" Reported by Abu Daawood (no. 4607), at-Tirmidhee (no. 2676) and it is saheeh. Ibn Hajr authenticated it Takhreej Ahaadeeth Ibn ul-Haajib (1/137).
And he (sallallaahu alaihi wasallam) also said: "… and every innovation is misguidance and all misguidance is in the Hellfire." Reported by an-Nasaa'ee (1/224) from Jaabir bin Abdullaah and it is saheeh as declared by Shaikh ul-Islaam Ibn Taymiyyah in Majmoo' ul-Fataawaa (3/58).
The Messenger (sallallaahu alaihi wasallam) also warned against the People of Innovation, from befriending, supporting or taking from them saying: "Whoever innovates or accommodates an innovator then upon him is the curse of Allaah, His Angels and the whole of mankind." Reported by Bukhaaree (12/41) and Muslim (9/140)
And in his footsteps, we find the Noble Companions and the Taabi'een after them warning from the danger of innovations upon the Ummah, its people and their unity, since it is innovations which have divided the Ummah and split it asunder.
Ibn Abbaas (d. 68H) said: "Indeed the most detestable of things to Allaah are the innovations." Reported by al-Bayhaqee in as-Sunan al-Kubraa (4/316)
Ibn Umar (d. 84H) said: "Every innovation is misguidance, even if the people see it as something good." Reported by Abu Shaamah (no. 39)