Yahya related to me from Malik from Nafi that Abdullah ibn Umar, when questioned about forgetfulness in the prayer, said, "If you think that you have forgotten part of the prayer, then pray it."
أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ كَانَ إِذَا سُئِلَ عَنِ النِّسْيَانِ فِي الصَّلاَةِ قَالَ لِيَتَوَخَّ أَحَدُكُمُ الَّذِي يَظُنُّ أَنَّهُ نَسِيَ مِنْ صَلاَتِهِ فَلْيُصَلِّهِ
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."
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