I forgot a prostration; what should I do?
Short answer
Prostration is a pillar; whoever is certain he left it does not suffice with the prostration of forgetfulness alone, but does what is required to make up the pillar according to where he remembers.
Evidence and explanation
In the case mentioned the Shaykh ruled to perform a complete rak'ah after being reminded and then the prostration of forgetfulness; which shows that the omitted prostration is a pillar that is not made up by the prostration of forgetfulness alone.
Scholar referenced
This editorial summary is based on the original source linked below and is not presented as a verbatim quotation from the scholar.
From the text of the answer (Arabic, with an English translation)
Original answer — Shaykh ‘Abd al-‘Aziz ibn Baz
حكم صلاة مَن سجد سجدةً واحدةً فقط ناسيًا — من ضمن أسئلة تابعة لتعليق سماحته على محاضرة بعنوان (الصلاة وأهميتها) في الجامع الكبير بالرياض. (مجموع فتاوى ومقالات الشيخ ابن باز ١١/ ٢٧٧).- Question
«إمام صلَّى صلاة العصر، وأثناء الركعة الأخيرة سجد سجدةً واحدةً فقط، وجلس للتشهد الأخير وسلَّم، وبعد ذلك نبَّهه بعضُ المصلين إلى أنه لم يسجد إلا سجدةً واحدةً، ولم يسجد الثانية للركعة الأخيرة، ثم قام وصلَّى بهم ركعةً كاملةً، وجلس للتشهد، ثم سلَّم وسجد سجود السهو، هل هذه الصفة صحيحة؟»
English translation (unreviewed): An imam prayed the 'Asr prayer, and during the last rak'ah he prostrated only once, sat for the final tashahhud and gave the salam. Afterwards some worshippers alerted him that he had prostrated only once and had not made the second prostration of the last rak'ah. Then he stood and led them in a complete rak'ah, sat for the tashahhud, gave the salam and performed the prostration of forgetfulness. Is this manner correct?
- From the text of the answer
«نعم، هذا هو المشروع، إذا نسي الإمام سجدةً وسلَّم ثم ذكر أو نُبِّه، يقوم ويأتي بركعةٍ، ثم يُكمل، ثم يُسلِّم، ثم يسجد سجود السهو بعد السلام، وهو أفضل، وهكذا المنفرد حكمه حكمه، وإن سجد للسهو قبل السلام فلا بأس، ولكن بعده أفضل [١] .»
English translation (unreviewed): Yes, this is what is prescribed. If the imam forgets a prostration and gives the salam, then remembers or is alerted, he stands and performs a rak'ah, then completes, then gives the salam, then performs the prostration of forgetfulness after the salam, which is better. The one praying alone has the same ruling. If he performs the prostration of forgetfulness before the salam there is no harm, but after it is better [1].
The Arabic excerpts matched the original page word for word by an automated check and have not had human scholarly review. The English translations are unreviewed translations made for this site; they are not an approved Shari‘ah translation. Rely on the Arabic original.
Source
Ruling on the prayer of one who prostrated only once out of forgetfulness
Title of the original page (Arabic): حكم صلاة مَن سجد سجدةً واحدةً فقط ناسيًا
Shaykh ‘Abd al-‘Aziz ibn Baz, may Allah have mercy on him