What is the meaning of "investigation" (al-taharri) in the prostration of forgetfulness (sujood al-sahw), and what is required of a woman if she conceals her voice during a prayer meant to be recited aloud, or recites aloud during a prayer meant to be recited silently?

The command for "investigation/deliberation" (al-taharri) appears in the hadith of Ibn Mas'ud, which is authentically narrated in the Two Sahihs. Scholars have differed on its meaning:

1. The majority (Malik, Al-Shafi'i, and a narration from Ahmad): They hold that "al-taharri" means to build upon the certain minimum, relying on the hadith of Abu Sa'id, "Let him cast away doubt and build upon what he is certain of."

2. Abu Hanifa and a narration from Ahmad chosen by Ibn Taymiyyah: They believe that "al-taharri" is to act based on the predominant probability (ghalabat al-dhann). If the doubt is evenly balanced, one builds upon the minimum. However, if one of the possibilities predominates in one's mind, one acts upon it. They cite as evidence the difference in the positions of the prostration of forgetfulness (sujud al-sahw) between the hadith of Ibn Mas'ud (after the salutation) and the hadith of Abu Sa'id (before the salutation).

As for a woman's audibility (jahr) and inaudibility (israr) in prayer, the correct view is that she is like a man: she vocalizes where vocalization is prescribed and whispers where whispering is prescribed, due to the generality of the divine address. However, if there is a non-mahram man present, she should whisper to prevent temptation (fitnah). Vocalizing and whispering in their respective places are sunnah, and the prayer is valid in all cases. Scholars have differed on the permissibility of the prostration of forgetfulness for one who forgets and vocalizes in a place where whispering is prescribed or vice versa:

Al-Nawawi, Al-Awza'i, and Ahmad in the most authentic of the two narrations: The prayer is not invalidated, and there is no prostration of forgetfulness for it, but one has committed a disliked act (makruh). Malik, Al-Thawri, Abu Hanifa, and Ishaq: One should perform the prostration of forgetfulness.