The Quran

إِنَّمَا المُؤمِنونَ الَّذينَ آمَنوا بِاللَّهِ وَرَسولِهِ وَإِذا كانوا مَعَهُ عَلىٰ أَمرٍ جامِعٍ لَم يَذهَبوا حَتّىٰ يَستَأذِنوهُ ۚ إِنَّ الَّذينَ يَستَأذِنونَكَ أُولٰئِكَ الَّذينَ يُؤمِنونَ بِاللَّهِ وَرَسولِهِ ۚ فَإِذَا استَأذَنوكَ لِبَعضِ شَأنِهِم فَأذَن لِمَن شِئتَ مِنهُم وَاستَغفِر لَهُمُ اللَّهَ ۚ إِنَّ اللَّهَ غَفورٌ رَحيمٌ

Muhammad Asad

[TRUE] BELIEVERS are only they who have attained to faith in God and His Apostle, and who, whenever they are [engaged] with him upon a matter of concern to the whole community,¹ do not depart [from whatever has been decided upon] unless they have sought [and obtained] his leave.² Verily, those who [do not abstain from the agreed-upon action unless they] ask leave of thee - it is [only] they who [truly] believe in God and His Apostle! Hence, when they ask leave of thee for some [valid] reason of their own, grant thou this leave to whomsoever of them thou choose [to grant it],³ and ask God to forgive them: for, behold, God is muchforgiving, a dispenser of grace!⁴

Lit., "a uniting [or "collective"] matter" (amr jami'). The personal ptonoun in "with him" relates to the Apostle and, by analogy, to every legitimate leader (imam) of the Muslim community acting in accordance with the spirit of the Qur'an and the Prophet's life-example.
I.e., his permission to abstain, for valid reasons, from participating in a course of action or a policy agreed upon by the majority of the community ('amma ijtama'u lahu min al-amr: Tabari). In a logical development of this principle we arrive at something like the concept of a "loyal opposition", which implies the possibility of dissent on a particular point of communal or state policy combined with absolute loyalty to the common cause. (But see also note 91.)
I.e., after weighing the reasons advanced by the individual or the individuals concerned against the interests of the society as a whole.
The statement that "God is much-forgiving" obviously implies that an avoidance of "asking leave" to abstain from participation in an agreed-upon course of action is, under all circumstances, morally preferable (Zamakhshari)