صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب السهو فى الصلاةChapters on Forgetfulness in Prayer
(428) بَابُ ذِكْرِ الْمُصَلِّي يَشُكُّ فِي صَلَاتِهِ وَلَهُ تَحَرٍّ،The chapter on a praying person doubting his prayer and making an effort to determine
Hadith Q1028
وَالْأَمْرِ بِالْبِنَاءِ عَلَى التَّحَرِّي إِذَا كَانَ قَلْبُهُ إِلَى أَحَدِ الْعَدَدَيْنِ أَمْيَلَ، وَكَانَ أَكْثَرُ ظَنِّهِ أَنَّهُ قَدْ صَلَّى مَا الْقَلْبُ إِلَيْهِ أَمْيَلُ
The statement regarding the command to base it on investigation and inquiry when his heart inclines more towards a certain number, and he predominantly thinks that the number his heart inclines towards is the number of prayers he has performed.
