The various actions that we perform and which are related to our duty (to Allah) are divided into five categories: wajib, haram, mustahab, makruh, and mubah. This classification is based on certain criteria existing in every Islamic rule. The criterion for an obligatory rule (wajib) is a strong expediency that has reached its pinnacle point and which cannot be skipped. The criterion for the rule of prohibition (haram) is an extreme evil which the Legislator (God or His Prophet (S)) does not allow us to do. The criterion for a recommended rule (mustahab) is an expediency which has not reached the pinnacle point and it is therefore such that it can be skipped, although it is better to do it. The criterion for the rule of abominableness or undesirability (makruh) is an evil which has not reached a high degree of obscenity and it is, therefore, permissible to do it but it is better not to do it. As for mubah, it is a rule which is neither preferred to do nor preferred to abandon. According to the Legislator, there is no preference in doing or skipping it.
Obviously, if there is a reliable and authentic narration which explicitly state that something is desirable[1] or makes reference to the degree of something being good,[2] it would then be recommended to perform that act but it is necessary to note that if a narration with a poor chain of transmission indicates that something incurs reward, it can be said with the help of "man-balag" reports that God shall give the doer of that act the reward promised in the narration, even though the desirability of that act may not be proved due to the report being poor.
To make a long story short, there is a principle known among jurisprudents and other Muslim scholars as "tasamuh fi adillah al-Sunan"[3] (compromise in the arguments regarding prophetic traditions). This principle is derived from authentic and well-known reports existing in our textual sources and known as "akhbar man balagh"[4] or reports acted upon with the intention of gaining reward. When it comes to the purport of the narration, the jurisprudents have different views. The technical discussion concerning the narration should take place in its respective place but we would say briefly that some scholars are of the view that if the 'man-balagh' reports are added to these poor reports, the desirability of this action is proved albeit with the argument that 'man-balagh' reports prove the reward for the deeds which weak reports claim to have reward. Such an alleged reward is also a sign of being desirable and recommended.[5] Some others accept the significations of these narrations about having reward only. They say: "The purport of these reports is that God gives the doer of this act the reward out of His Grace not because that act actually becomes recommended.[6]
Therefore, the narrations which have been mentioned in du'a and ziarat books and which encourage us to do certain acts, for example, it has been said that if a person recites a certain du'a (supplication) during wudhu, or recites a certain zikr (recital) before going to bed, or recites a certain du'a on the night of Qadr or say such and such words while visiting the graves of the Imams (AS), he will get so and so reward; In such cases, there is no problem in acting upon such narrations. In fact, based on the judgment of sound reason and authentic narrations, the doer will be treated with benevolence and grace.  Of course, two conditions should be met in order for him to act according to narrations and receive the reward. One, there should be no reason or evidence showing that the action is haram or abominable. Second, one should perform that act with the intention of attaining its reward.[7]

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