The Prophet ﷺ said: Anyone who is in doubt in his prayer should make two prostrations after giving the salutation. (Using translation from Abū Dāʾūd 1033)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ مَنْ شَكَّ فِي صَلاتِهِ فَلْيَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يُسَلِّمُ