So, when you meet those who disbelieve strike at their necks till when you have killed and wounded many of them, then bind a bond firmly (i.e. take them as captives). Thereafter (is the time) either for generosity (i.e. free them without ransom), or ransom until the war lays down its burden (Muhammad 4).
Aqsaam (categories Of) Al-waajib In Regards To The Consideration Of The Specification Of What Is Required And Its Non-specification
1 – Al-Waajib Al-Mu’ayyan (specified):
This is what the Shaari’ has requested in itself without providing the choice to the Mukallaf to choose between different matters (or actions). The Salaah is an example of such an obligation. As such the (obligation of the) Salaah does not fall from the Mukallaf unless he has performed it in itself, so it cannot be substituted by the recitation of the Qur’aan and fasting.
2 – Al-Waajib Ghair ul-Mu’ayyan (non-specified) or Al-Waajib Al-Mukhayyar (of choice): This is what the Shaari’ (Legislator) has requested to be undertaken; not in itself (specifically) but within matters which have been made known. It is up to the Mukallaf to choose one from them in order to perform and fulfil such a Waajib. This Waajib could be one of two like in the Qawl of Allah Ta’Aalaa:
This informs us that it is up to the Khalifah either to let the captives of war free or to exchange them by ransom.
The non-specified Waajib could also be one from three matters and its example is the Kaffaarah (expiation) of the Yameen (oath). It is obligatory upon the one who breaks the oath to do one of three things: The feeding of ten poor, or providing clothing to them or to free a slave. This is in the case when the capability exists but if the capability is not present then the obligation is Mu’ayyan (specified) and that is the fasting of three days.