بُلُوغُ الْمَرَامِ
Bulugh al-Maram 336
The Book of Prayer
وَلِأَحْمَدَ , وَأَبِي دَاوُدَ , وَالنَّسَائِيِّ ; مِنْ حَدِيثِ عَبْدِ بْنِ جَعْفَرٍ مَرْفُوعاً : { مَنْ شَكَّ فِي صَلَاتِهِ , فَلْيَسْجُدْ سَجْدَتَيْنِ بَعْدَمَا يُسَلِّمُ } وَصَحَّحَهُ اِبْنُ خُزَيْمَةَ . [1]
In the narration of Ahmad, Abu Dawud and an-Nasa'i from the Hadith of 'Abdullah bin Ja'far (RA) Marfu' [attributed to the Prophet(ﷺ)]: 'Whoever doubts about his Salat (prayer), should make two prostrations after the Taslim (salutations).' [Ibn Khuzaimah graded it Sahib (authentic)].
Sunnah.com reference: Book 2, Hadith 237