In equivocal cases the believer must seek clarification until the truth becomes obvious to him. For he may not embark upon an act of obedience without knowledge and intention, while if he leaves it undone he is a sinner, since it is sinful to ignore a necessary duty. It is also sinful to do what has been prohibited, however. The implications of all this are far-reaching and there is neither power nor strength except with God.
Under this category falls the concession made by the Prophet, on him be peace, to leading hypocrites and reprobates like Abdullah ibn Ubayy in view of their numerous support. For to have penalized him in some way for his wrong conduct would have entailed the loss of a greater good, through the fury and chauvinism of his tribesmen and through the aversion of people would have felt on hearing that Muhammad was killing his Companions.
"Public Duties in Islam" - Ibn Taymiyya, p. 81