In each of the four surahs ‘al-Sajdah’, ‘Fuṣṣilat’, ‘al-Najm’, and ‘al-ʿAlaq’, there is a verse of sajdah, which means that if one recites this verse or listens to it, he must immediately perform sajdah after the verse has finished. If he forgets to do this, he must perform sajdah whenever he remembers. Performing sajdah is not obligatory if one hears such a verse involuntarily, although it is better that he does.
Surah al-Sajdah (32:15)
Surah Fuṣṣilat (41:37)
Surah al-Najm (53:62)
Surah al-ʿAlaq (96:19)
Even if one does not say any dhikr when he places his forehead on the ground with the intention of performing an obligatory sajdah of the Qur’an, it is sufficient. However, saying a dhikr is recommended, and it is better to say the following:
لَا إِلٰهَ إِلاَّ اللهُ حَقّاً حَقّاً
There is no god but Allah, truly, truly
لَا إِلٰهَ إلاَّ اللهُ إِیمَاناً وَّ تَصْدِیقاً
There is no god but Allah, I believe in this certainly and I affirm it certainly
لَا إِلٰهَ إلاَّ اللهُ عُبُودِیَّةً وَّ رِقّاً
There is no god but Allah, I testify this in servitude and as a slave
سَجَدْتُ لَكَ یَا رَبِّ تَـعَبُّدًا وَّ رِقّاً
I prostrate to You, O my Lord, in servitude and as a slave
لَا مُسْتَـنْکِفاً وَّ لَا مُسْتَکْبِراً
not disdainfully nor arrogantly
بَلْ أَنَا عَبْدٌ ذَلِیلٌ ضَعِیفٌ خـَائِفٌ مُّسْتَجِیرٌ
Rather, I am a servant lowly, weak, fearing, and seeking refuge