In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Amr bin AI-Aas [radiyallaahu anhu] narrated that he heard Allaah’s Messenger [sallal laahu alayhi wasallam] saying, “If a Mujtahid gives a verdict according to the best of his knowledge and his verdict is correct, he will receive a double reward, and if he gives a verdict according to the best of his knowledge and his verdict is wrong, even then he will get a reward.
[Bukhari Number 7352]
Few Masaa’il From This Hadeeth
 A Mujtahid is not deprived of reward.
 The judgement of a non-Mujtahid is different from that of a Mujtahid. Shaikhul Islaam Ibn Taymiyyah [rahimahullaah] said that the one who speaks about the religion without knowledge and out of ignorance, then indeed he has lied and sinned. The one who intentionally speaks to oppose the truth, he is threatened with entry into the fire of hell, as opposed to the affair of the one who speaks based on permissible Ijtihaad, for indeed he strives, fears Allaah as much as he is able, desires to seek knowledge as much as he was able, speaks for the sake of Allaah, knows the stronger proof and speaks based on that. This person is given two rewards if he is correct; but if he is mistaken, he receives one reward.
 A Mujtahid can be correct or mistaken. [Reg 1]
Shaikh Rabee Bin Haadee [hafidha-hullaah] said:
We do not say that there are people of ijtihaad amongst ahlul bidah because they are followers of desires, and Allaah and His Messenger have testified to this. The misguided innovator stirs up discord and errs, and then says to you, ‘This is ijtihaad’. When Hikmatiyaar and the misguided parties [in Afghanistan] murdered Shaikh Jameel Ur-Rahmaan, they said: “This is ijtihaad.” They [ahlul bidah] do not fall into an affair of misguidance or a calamity except that they say, “This is ijtihaad.” This is [tantamount] to diluting Islaam, falsehood, misguidance, bidah and confounding truth with falsehood, when the mistakes of the [true and qualified scholars of ijtihaad]- for which they are rewarded- are placed at the same level with innovation, about which the Messenger [sallal-laahu-alayhi-wasallam] declared to be destined for the hell fire. [Ref 2]
[Ref 1] Source: Al-Arba’oona Hadeethaa Fee Usoolil Fiqh. pages 32-33. By Shaikh Muhammad Baazmool [hafidhahullaah]. slightly paraphrased
[Ref 2]Source: Ajwibatul Allaamatush Shaikh Rabee Bin Haadee Al Madkhalee Alaa As-ilah Abee Rawaaha Al-Manhajiyyah page: 20. abridged