The ruling on one who was in the tashahhud when the iqamah was called and he cut off the tashahhud from its middle and said the taslim
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Question : The second question: The one praying sat for the tashahhud, then the iqamah was called. The one praying recited for his tashahhud the following: "The greetings are for Allah, peace be upon the Prophet" Sunan ad-Daraqutni 3:400 then he said the taslim. What is the ruling on his tashahhud and his prayer?
Answer : What we hold is that this is not sufficient. If the iqamah is called while he is in the tashahhud, he should complete it, but quickly. As for omitting it, it is not valid. And on this occasion, I say: if the iqamah is called and you are in the first rak'ah, cut it off; and if you are in the second rak'ah, complete it lightly. Yes.
I bear witness that there is no god but Allah, and I bear witness that Muhammad is His slave and His Messenger. O Allah, send blessings upon Muhammad
أشهد أن لا إله إلا الله وأشهد أن محمدا عبده ورسوله اللهم صل على محمدAnswer : What we hold is that this is not sufficient. If the iqamah is called while he is in the tashahhud, he should complete it, but quickly. As for omitting it, it is not valid. And on this occasion, I say: if the iqamah is called and you are in the first rak'ah, cut it off; and if you are in the second rak'ah, complete it lightly. Yes.
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، [3725] حكم من كان في التشهد وأقيمت الصلاة فقطع التشهد من منتصفه وسلم -تعليقات على الكافي لابن عثيمين — youtube.com
The Arabic text is a machine transcription of the audio recording.