The Prophet ﷺ said: Anyone who is in doubt in his prayer should make two prostrations after giving the salutation.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ شَكَّ فِي صَلاَتِهِ فَلْيَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يُسَلِّمُ