كتاب السهو
Read the English translation first, with the original Arabic directly underneath.
It was narrated from Abu Sa'eed Al-Khudri that:
The Prophet (ﷺ) said: "If one of you does not know whether he prayed three or four (rak'ahs), let him pray a rak'ah then prostrate twice after that when he is sitting. Then if he prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the shaitan."
إِذَا لَمْ يَدْرِ أَحَدُكُمْ صَلَّى ثَلاَثًا أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً ثُمَّ يَسْجُدْ بَعْدَ ذَلِكَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعَتَا لَهُ صَلاَتَهُ وَإِنْ صَلَّى أَرْبَعًا كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ