to perform it, while it is called voluntary on a particular supposition: he is rewarded and recompensed for it; like voluntary worship, it is not wasted effort. In itself, it was not obligatory upon him, but became obligatory because of his doubt. Moreover, according to one of the two reports from Ahmad, he requires the prayer to be repeated with the local imam, yet calls it voluntary, in accordance with the Prophet’s command, peace and blessings be upon him. Likewise, in the hadith of Abū Dharr, he said:

“Perform the prayer at its proper time, then make your prayer with them voluntary, and do not say, ‘I have already prayed.’”

It is voluntary in the sense that it is additional to the five basic obligatory prayers, even if it becomes obligatory for another reason, as happens with an act made obligatory by a vow.

Many of the early generations used the term nāfilah (“additional worship”) for worship that increases one’s good deeds, which applies to someone who has no sins. Hence, concerning His words:

“And during part of the night, pray with it as additional worship for you,”

they said that this additional worship was exclusive to the Messenger of Allah, peace and blessings be upon him, because Allah had forgiven him, whereas others have sins which their prayers serve to expiate. Although this view calls for discussion, this is not the place for it. The point is that the term nāfilah has been used broadly: it may refer to something commanded, while its use may be denied for voluntary worship.

It has thus become clear that the prostrations of forgetfulness are obligatory. Their cause is either an omission or an addition, as he said in the two Sahīhs:

“If he adds something or omits something, let him perform two prostrations.”

An omission is illustrated by the hadith of Ibn Buhaynah: when he omitted the first tashahhud, he prostrated. As for an addition,