What is the legal ruling that prevents an ordinary person from returning to act upon a fatwa after abandoning it, and to what extent is it permissible to repeat this?

A layman must imitate the scholar he trusts. If the scholar issues a fatwa, and the layman acts upon it, then abandons acting upon it due to his whims, he is sinful. However, if he abandons acting upon the fatwa because of the saying of another scholar whom he finds more trustworthy, there is no blame upon him. His returning to the practice of the mufti whom he followed out of whim is obligatory, but he is not permitted to return to it where the more trustworthy scholar has issued a fatwa, because he is obligated to follow the most trustworthy person in his own estimation.