“The Satan comes between the son of Adam and his soul, and he does not know how many Rak’ah he has prayed. If a person notices that, then let him prostrate twice before he says the Salam.”  

أَنَّ النَّبِيَّ ﷺ قَالَ إِنَّ الشَّيْطَانَ يَدْخُلُ بَيْنَ ابْنِ آدَمَ وَبَيْنَ نَفْسِهِ فَلاَ يَدْرِي كَمْ صَلَّى فَإِذَا وَجَدَ ذَلِكَ فَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ  

[Machine] From the Prophet, ﷺ , he said: Indeed, Satan comes to one of you during his prayer, and he does not know whether he has increased or decreased [his prayer]. So, when one of you finds that, let him perform two prostrations.  

عَنِ النَّبِيِّ ﷺ قَالَ إِنَّ الشَّيْطَانَ يَأْتِي أَحَدَكُمْ فِي صَلَاتِهِ فَلَا يَدْرِي أَزَادَ أَمْ نَقَصَ فَإِذَا وَجَدَ أَحَدُكُمْ ذَلِكَ فَلْيَسْجُدْ سَجْدَتَيْنِ  

Messenger of Allah ﷺ said, "When anyone of you stands for the prayers, Satan comes and puts him in doubts till he forgets how many rakʿat he has prayed. So if this happens to anyone of you, he should perform two prostrations of Sahu while sitting. (Using translation from Bukhārī 1232)   

"إِنَّ الشَّيطَانَ يَأْتِي أَحَدَكُمْ في صَلاته فَيُلْبِسُ عليه حَتَّى لا يَدْرى كَمْ صَلَّى. فَإِذَا وَجَدَ ذَلكَ أَحَدُكُمْ فَليَسْجُدْ سَجْدَتَينِ وَهُوَ جَالِسٌ قَبْلَ أن يُسَلَّمَ، ثُمَّ يُسَلِّمْ".  

"إِذا شَكَّ أحدُكُمْ في صَلاتِه فلم يَدْر زادَ أوْ نَقَص؟ فإِنْ كانَ شكّ في الواحدةِ وَالثِّنتين فليجْعلها واحدةً حتَّى يكونَ الوهْمُ في الزِّيادةِ ثمَّ يَسْجُدْ سجدتينِ وهو جالسٌ قَبْل أنْ يُسَلِّمَ ثُمَّ يُسَلِّمْ ".