He did the first sajdah of sujood al-sahw (prostrations for forgetfulness in the prayer), then he remembered that he did not do anything that required sujood al-sahw
Praise be to Allaah.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked this question, and he hesitated, then he replied:
The best thing is for him to do the second sajdah, so as not to confuse people and give them cause to shout “Subhaan Allaah” [to draw his attention to his mistake] – that is, if he is the imaam. And Allaah knows best.