Hadith 213
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عُمَرَ بْنِ مُحَمَّدِ بْنِ زَيْدٍ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَوَخَّ الَّذِي يَظُنُّ أَنَّهُ نَسِيَ مِنْ صَلاَتِهِ فَلْيُصَلِّهِ ثُمَّ لِيَسْجُدْ سَجْدَتَىِ السَّهْوِ وَهُوَ جَالِسٌ .
Yahya related to me from Malik from Umar ibn Muhammad ibn Zayd from Salim ibn Abdullah that Abdullah ibn Umar used to say, "If you are uncertain in the prayer, estimate what you think you have forgotten of the prayer and repeat it, then do the two sajdas of forgetfulness from the sitting position
Mauquf Sahih (Salim al-Hilali)
Muwatta Malik 213 · Book 3, Hadith 46