"When a judge exercises ijtihad and gives a right judgment, he will have two rewards, but if he errs in his judgment he will still have earned one reward." [Abu Dawud]
This hadith evidently encourages the spirit of tolerance in ijtihad by promising a reward even for one who might have inadvertently fallen into error. Since the hadith has taken a positive view of such effort, scholars and mujtahids are also required to exercise restraint in denouncing a view which they might consider erroneous.
This hadith also lends support to the conclusion that a judicial decision that is made in the true spirit of ijtihad is enforceable and the judge may not be taken to task for it if it later turns out that he had made an error of judgment. Similarly, when a person trusts the integrity and knowledge of a scholar of Shariah and acts upon his verdict (fatwa) on a legal question but later discovers that the fatwa was erroneous, he would have committed no wrong, simply because the hadith exonerates an error of that kind in the first place. The reward that is promised is, however, earned only by those whose sincerity and devotion to a good cause are not in question.
"Shariah Law - An Introduction" - Mohammad Hashim Kamali, p. 108