Is there evidence from the Qur'an or Sunnah that the Ummah will not agree upon error, or for the authoritativeness of Ijma' (consensus) as a source of legislation?

Consensus (Ijma') is a source and a decisive proof for Islamic scholars. Its proofs are numerous, including the Almighty's saying: (And whoever contradicts the Messenger after guidance has become clear to him and follows other than the way of the believers – We will give him what he has taken and drive him into Hell, and evil it is as a destination) and His saying: (And if you disagree over anything, refer it to Allah and the Messenger). Also, the Hadith: "This Ummah will not agree upon error," which is supported by the meaning of the two verses.