A man stood up for the fifth [rak'ah] and remembered before he began the recitation, what
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The questioner : A man stood up for the fifth [rak'ah]. He remembered before he began the recitation. What do you say?
The Shaykh : Yes, he must go back to the place of the delay.
The questioner : All right, he began the recitation?
The Shaykh : And, if Allah wills, because it is an extra rak'ah.
The questioner : Do we say it is like the first tashahhud?
The Shaykh : The first [tashahhud] is obligatory. Yes. Also, the tashahhud is a deficiency, whereas this is an addition. Some imams were ignorant and thought that if a person stands up for an extra [rak'ah] and begins the recitation, it is forbidden for him to go back, and this is an error. This [ruling] applies only to the one who stands up from the first tashahhud,
The Shaykh : Yes, he must go back to the place of the delay.
The questioner : All right, he began the recitation?
The Shaykh : And, if Allah wills, because it is an extra rak'ah.
The questioner : Do we say it is like the first tashahhud?
The Shaykh : The first [tashahhud] is obligatory. Yes. Also, the tashahhud is a deficiency, whereas this is an addition. Some imams were ignorant and thought that if a person stands up for an extra [rak'ah] and begins the recitation, it is forbidden for him to go back, and this is an error. This [ruling] applies only to the one who stands up from the first tashahhud,
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 2156رجل قام إلى خامسة وذكر قبل أن يشرع في القراءة ماذا..📔 الشرح الممتع على زاد المستقنع - ابن عثيمين — youtube.com
The Arabic text is a machine transcription of the audio recording.