Sunan Abi Dawud 1026
سنن أبي داود#1026Sahihحَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ عَطَاءِ بْنِ يَسَارٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلاَ يَدْرِي كَمْ صَلَّى ثَلاَثًا أَوْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً وَيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ التَّسْلِيمِ فَإِنْ كَانَتِ الرَّكْعَةُ الَّتِي صَلَّى خَامِسَةً شَفَعَهَا بِهَاتَيْنِ وَإِنْ كَانَتْ رَابِعَةً فَالسَّجْدَتَانِ تَرْغِيمٌ لِلشَّيْطَانِ " .
Narrated Ata' ibn Yasar: The Prophet (ﷺ) said: When one of you is in doubt about his prayer, and does not know how much he has prayed, three or four rak'ahs, he should pray one (additional) rak'ah and make two prostrations while sitting before giving the salutation. If the (additional) rak'ah which he prayed is the fifth one, he will make it an even number by these two prostrations. If it is the fourth one, the two prostrations will be a disgrace for the devil
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 2:637