I recited before the Prophet (may Allah's peace and blessings be upon him) Sūrat An-Najm and he did not prostrate in it

Zayd ibn Thābit (may Allah be pleased with him) recited Sūrat An-Najm before the Prophet (may Allah's peace and blessings be upon him) and when he recited the verse of prostration, the Prophet didn't prostrate. The Prophet's action does not indicate that prostration should be left out all the time, because it is possible that he did not prostrate to show that leaving prostration is permissible. This is the preponderant opinion, which is concluded by Imām Ash-Shāfi‘i.

Had prostration been obligatory, the Prophet (may Allah's peace and blessings be upon him) would have ordered him to do it even later on.