A man prayed Maghrib behind an imam who was praying 'Isha, so he sat in the third rak'ah and said the taslim, then he prayed one rak'ah of 'Isha with the imam!
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Question:
He says: Honorable Shaykh, may Allah grant you success, I came to you while you were praying 'Isha, and in my mind I thought you were praying Maghrib, so I joined you. When you stood up for the fourth rak'ah, I stood up with you. When I straightened up, I remembered and went back and sat down. Then I said:
Answer:
The Maghrib prayer is valid. As for you standing up and praying one rak'ah of 'Isha, meaning you divided the prayer between two prayers, this is not valid. It is not permissible to divide the prayer between Maghrib and 'Isha. Yes.
He says: Honorable Shaykh, may Allah grant you success, I came to you while you were praying 'Isha, and in my mind I thought you were praying Maghrib, so I joined you. When you stood up for the fourth rak'ah, I stood up with you. When I straightened up, I remembered and went back and sat down. Then I said:
Answer:
The Maghrib prayer is valid. As for you standing up and praying one rak'ah of 'Isha, meaning you divided the prayer between two prayers, this is not valid. It is not permissible to divide the prayer between Maghrib and 'Isha. Yes.
Source
قناة «العلامة الدكتور صالح الفوزان»، [827 -2802] رجل صلى المغرب وراء إمام يصلي العشاء فجلس في الثالثة وسلم ثم صلى العشاء مع الإمام ركعة!! — youtube.com
The Arabic text is a machine transcription of the audio recording.