What is the ruling on someone who, after the second unit (rak'ah) of a four-unit prayer, mistakenly said the taslim (salutation of peace), then said the takbir (Allahu Akbar) for the supererogatory (ratibah) prayer, and upon remembering the incomplete obligatory prayer, intended both the obligatory and supererogatory as two voluntary prayers, and then repeated both the obligatory and supererogatory prayers?

Whoever forgets and makes the Tasleem before completing their prayer, then remembers shortly after, should return and complete their prayer, and perform the prostration of forgetfulness (sujood al-sahw), as the Prophet, peace and blessings be upon him, did when he made the Tasleem after two rak'ahs, then completed his prayer and performed the two prostrations of forgetfulness. If one begins a voluntary (nafilah) prayer and then remembers that they have not completed an obligatory (fard) prayer, they should interrupt the voluntary prayer, complete the obligatory prayer, and perform the prostration of forgetfulness.

However, if a long time has passed, or one's ablution (wudu') has been invalidated, they must restart the prayer. If they do not remember until after beginning another prayer, then if what was done in the second prayer was little and not much time had passed, they should return to the first (incomplete) prayer and complete it. Otherwise, the first prayer becomes void. The determination of whether the interval is "long" or "short" is based on common understanding and custom. It is not permissible to consider the incomplete obligatory prayer as a voluntary one; rather, it is obligatory to complete it and then perform the prostration of forgetfulness.