Hadith 502250
Chapter 4: Forgetfulness in Prayer - كتاب السهو
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَجُلاً، سَأَلَ الْقَاسِمَ بْنَ مُحَمَّدٍ فَقَالَ إِنِّي أَهِمُ فِي صَلاَتِي فَيَكْثُرُ ذَلِكَ عَلَىَّ . فَقَالَ الْقَاسِمُ بْنُ مُحَمَّدٍ امْضِ فِي صَلاَتِكَ فَإِنَّهُ لَنْ يَذْهَبَ عَنْكَ حَتَّى تَنْصَرِفَ وَأَنْتَ تَقُولُ مَا أَتْمَمْتُ صَلاَتِي .
Yahya related to me from Malik that he had heard that a man questioned al-Qasim ibn Muhammad saying, "My imagination works in the prayer, and it happens to me a lot." Al-Qasim ibn Muhammad said, "Go on with your prayer, for it will not go away from you until you go away saying, 'I have not completed my prayer.' "
Reference Muwatta Malik 502250
USC-MSA web (English) reference : Book 4, Hadith 3