Mishkat al-Masabih #954
Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «إِن أحدكُم إِذا قَامَ يُصَلِّي جَاءَهُ الشَّيْطَان فَلبس عَلَيْهِ حَتَّى لايدري كَمْ صَلَّى؟ فَإِذَا وَجَدَ ذَلِكَ أَحَدُكُمْ فَلْيَسْجُدْ سجدين وَهُوَ جَالس»
Abu Huraira reported God's Messenger as saying, “When one of you stands up to pray the devil comes to him and confuses him so that he does not know how much he has prayed. If any of you has such an experience he should perform two prostrations during the period when he sits.”*
* i.e. before pronouncing the taslim.
(Bukhari and Muslim.)
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About Mishkat al-Masabih
This hadith is #954 from Mishkat al-Masabih, in the Book of Prayer.