Muwatta Malik 224

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 224

In-book

Book 4, Hadith 3