Allah the Exalted said,

وَمِنَ اللَّيْلِ فَسَبِّحْهُ ...

And during a part of the night glorify His praises,

meaning pray to Him.

Allah said in another Ayah,

وَمِنَ الَّيْلِ فَتَهَجَّدْ بِهِ نَافِلَةً لَّكَ عَسَى أَن يَبْعَثَكَ رَبُّكَ مَقَاماً مَّحْمُودًا

And in some parts of the night offer the Salah with it, as an additional prayer for you. It may be that your Lord will raise you to Maqam Mahmud. (17:79)

Ibn Abi Najih reported that Mujahid said that Ibn Abbas said that,

... وَأَدْبَارَ السُّجُودِ ﴿٤٠﴾

and after the prostrations.

refers to Tasbih, i.e. glorifying Allah's praises, after the prayers.

There is a Hadith collected in the Two Sahihs that supports this meaning.

Abu Hurayrah said,

"Some poor migrants came and said, `O Allah's Messenger! The wealthy people will get higher grades and will have permanent enjoyment.'

The Prophet said,

وَمَا ذَاكَ؟

Why is that?

They said, `They pray as we do, fast as we do, yet they give charity, but we can not. They free slaves, but we can not.'

The Prophet said,

أَفَلَا أُعَلِّمُكُمْ شَيْئًا إِذَا فَعَلْتُمُوهُ سَبَقْتُمْ مَنْ بَعْدَكُمْ وَلَا يَكُونُ أَحَدٌ أَفْضَلَ مِنْكُمْ إِلَّا مَنْ فَعَلَ مِثْلَ مَا فَعَلْتُمْ؟

*

تُسَبِّحُونَ
*

وَتَحْمَدُونَ
*

وَتُكَبِّرُونَ

دُبُرَ كُلِّ صَلَاةٍ ثَلَاثًا وَثَلَاثِين

Shall I tell you about something that, if you did it, you would catch up with those who have surpassed you and nobody would be better than you except those who would do the same Say,

*

`Subhan Allah,
*

Alhamdulillah and
*

Allahu Akbar,'

thirty-three times each after every prayer.

Later, they came back and said, `O Allah's Messenger! Our brethren, the wealthy Muslims, heard of what we did and they also did the same.'

The Prophet said,

ذلِكَ فَضْلُ اللهِ يُؤْتِيهِ مَنْ يَشَاء

This is a favor and grace of Allah, and He grants it to whom He wills.''

There is another way of explaining the Ayah.

It is that Allah's statement, وَأَدْبَارَ السُّجُودِ (and after the prostrations),

refers to the two Rak`ahs after the Maghrib prayer.

This was reported from Umar bin Al-Khattab, Ali bin Abi Talib and his son Al-Hasan, Abdullah bin Abbas, Abu Hurayrah and Abu Umamah, may Allah be pleased with them.

This is also the saying of Mujahid, Ikrimah, Ash-Sha`bi, An-Nakha`i, Al-Hasan Al-Basri, Qatadah, and others.