Which is the third source of deriving Islamic knowledge?

After the Quran of Allah and the Sunnah of the Prophet Muhammad (PBUH), the Ijmaa’ (Consensus of the Scholars) is the third source when deriving Islamic knowledge. The proofs of this in the Quran and Sunnah are many. Of them, the statement of  Allah, the Exalted, in the Quranic chapter actually named 

Ash- Shoorah (Mutual Consultation). Therein, Allah desribes the righteous believers in verse 42:38 saying:

{And those who have responded to their Lord and established prayer and whose affair is [determined by] consultation among themselves, and from what We have provided them, they spend}

Also, in the Quranic chapter An-Nisa 4:115:

And whoever defies the Messenger after guidance has become clear to them and follows a path other than that of the believers, We will let them pursue what they have chosen, then burn them in Hell—what an evil end!.

Also, the authentic statement of the Prophet (PBUH):

“'My nation will never collectively agree upon falsehood.' 

The discussion of scholars regarding the principles in consensus is extensive and detailed. It suffices to be certain that the scholars' conclusions all have as foundations what is available to them from the Quran and the Sunnah. In this regard, Imam Malik said in his Muwatta':

"As for 'the agreed-upon practice', it is something that the people of fiqh and knowledge agree upon without dispute. This is the agreement of the people of this community who contract agreements (ahl al-hall wa'l-'aqd) in any matter. By agreement we mean agreement in word, action or belief…."