Muwatta Malik #214
Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَفِيفِ بْنِ عَمْرٍو السَّهْمِيِّ، عَنْ عَطَاءِ بْنِ يَسَارٍ، أَنَّهُ قَالَ سَأَلْتُ عَبْدَ اللَّهِ بْنَ عَمْرِو بْنِ الْعَاصِ وَكَعْبَ الأَحْبَارِ عَنِ الَّذِي، يَشُكُّ فِي صَلاَتِهِ فَلاَ يَدْرِي كَمْ صَلَّى أَثَلاَثًا أَمْ أَرْبَعًا فَكِلاَهُمَا قَالَ لِيُصَلِّي رَكْعَةً أُخْرَى ثُمَّ لِيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ .
Yahya related to me from Malik from Afif ibn Amr as-Sahmi that
Ata ibn Yasar said, "I asked Abdullah ibn Amr ibn al-As and Kab al
Ahbar about someone who was uncertain in his prayer, and did not know
whether he had prayed three or four rakas. Both of them said, 'He
should pray another raka and then do two sajdas from the sitting
position.' "
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About Muwatta Malik
This hadith is #214 from Muwatta Malik, in the Book of Prayer.