Muwatta Malik 215

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، 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 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

Reference

Muwatta Malik 215

In-book

Book 3, Hadith 48