Ijtehad in shari'a, allowed?

What do ahadith say about ijtihad?Ayatullah Murtadha Mutahhiri says in his book "The Role of Ijtihad in Legislation"

"Apparently, there seems to be no doubt that this term was never used by any of the Imams of the Ahl al-Bayt (A). The terms ijtihad and mujtahid, in the sense in which they are used by Shi'ah and Sunni fuqaha', have not been used in any of their ahadith. Neither they themselves were ever known by the epithet 'mujtahid' nor did they ever use it for the scholars and legists from among their companions." 

http://www.al-islam.org/al-tawhid/default.asp?url=ijtihad-legislation.htm    
  So as we can see, this term was never even mentioned in ahadith, which would indicate that it was imported from somewhere else and has unfortunately now become the focal point of religion. So since we do not have ahadith about ijtehad, let's see what our classical scholars said about it.        

Uddat al usul (By Shaikh Tusi)

اعلم ان هذه المسألة تسقط عن اصولنا، لانا قد بينا ان القياس والاجتهاد لا يجوز استعمالها في الشرع

I know (believe) that these issues are redundant about our principles, for I certainly have said/explained that the qiyas (analogy) and the ijtehad are both non permissible for use in shari'a.

Source

Uddat ul usul (by Shaikh Tusi), Vol 2, page 733

Retrieved online from:

http://www.yasoob.com/books/htm1/m018/22/no2209.html