Pacta sunt servanda - agreements must be obeyed - is a peremptory principle of modern international law of treaty. What are the origin and nature of this doctrine? Some say, its Latin wording indicates that it is of Roman origin. And this doctrine is a must for the social good; without it the society will be a chaos. But how does Islam perceive the notion? This paper finds that this doctrine came into being with the very beginning of the creation of human souls by Allaah Whom they accepted as their Sole Lord and made a commitment to follow His commands. And it is a principal tenet for Muslims to adhere to because it is an inseparable part of their faith (Iman), a command from their Lord and a practice (Sunnah) of their Prophet Muhammad (peace be upon him). It is, therefore, obligatory not merely because it is good for the society, but is also rooted in the teachings of Islamic doctrine and a Muslim’s accountability in the life hereafter.

Custom is a source of Islamic law in general and Siyar (Islamic international law) in particular. Islamic jurists have set out the elements and conditions of customs for general jurisprudential purpose. However, no one has, to the authors’knowledge, formulated them from Siyar perspective. This paper is an attempt to fill this gap by tracing two important elements of an international custom, namely frequent and dominant general practice of States, and acceptance of that practice as law. These two will constitute a valid custom provided they fulfill certain conditions, most importantly that the custom must not conflict with Shari’ah or the spirit of Shari’ ah.