The Tahajjud was Compulsory

بِسۡمِ ٱللهِ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ 

The following verses were Revealed first: 

سُوۡرَةُ بنیٓ اسرآئیل / الإسرَاء

أَقِمِ ٱلصَّلَوٰةَ لِدُلُوكِ ٱلشَّمۡسِ إِلَىٰ غَسَقِ ٱلَّيۡلِ وَقُرۡءَانَ ٱلۡفَجۡرِ‌ۖ إِنَّ قُرۡءَانَ ٱلۡفَجۡرِ كَانَ مَشۡہُودً۬ا (٧٨) وَمِنَ ٱلَّيۡلِ فَتَهَجَّدۡ بِهِۦ نَافِلَةً۬ لَّكَ عَسَىٰٓ أَن يَبۡعَثَكَ رَبُّكَ مَقَامً۬ا مَّحۡمُودً۬ا (٧٩) 

Establish regular prayers at the sun's decline until the darkness of the night, and the morning prayer and reading: for the prayer and reading in the morning carry their testimony.  And pray in the small watches of the morning: (it would be) an additional prayer (or spiritual profit) for thee: soon will thy Lord Raise thee to a station of praise and glory! (Surah al-Isra’:78-79) 

The night vigil is a Comfort and a Blessing that Allah (s.w.t.) has Given to us.  So great is its Blessing, that it was compulsory in the beginning.  When the first portion of Surah al-Muzzammil was Revealed, it was taken as compulsory by the Prophet Muhammad (s.a.w.) and the companions. 

سُوۡرَةُ المُزمّل

بِسۡمِ ٱللهِ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ

يَـٰٓأَيُّہَا ٱلۡمُزَّمِّلُ (١) قُمِ ٱلَّيۡلَ إِلَّا قَلِيلاً۬ (٢) نِّصۡفَهُ ۥۤ أَوِ ٱنقُصۡ مِنۡهُ قَلِيلاً (٣) أَوۡ زِدۡ عَلَيۡهِ وَرَتِّلِ ٱلۡقُرۡءَانَ تَرۡتِيلاً (٤) 

In the Name of Allah, the Beneficent, the MercifulO thou folded in garments!  Stand (to pray) by night, but not all night ― half of it ― or a little less or a little more; and recite the Qur’an in slow, measured rhythmic tones. (Surah al-Muzzammil:1-4) 

Allah (s.w.t.) Kept Back the last portion of the surah for twelve months before Revealing it.  The tafsir according to a hadits from Swahih Muslim, on the authority of ‘Aishah (r.a.), is that is later became sunnah.  The one wrapped or folded in garments, the Prophet (s.a.w.), is enfolded in the Divine Attributes. 

سُوۡرَةُ المُزمّل

إِنَّ هَـٰذِهِۦ تَذۡڪِرَةٌ۬‌ۖ فَمَن شَآءَ ٱتَّخَذَ إِلَىٰ رَبِّهِۦ سَبِيلاً (١٩) ۞ إِنَّ رَبَّكَ يَعۡلَمُ أَنَّكَ تَقُومُ أَدۡنَىٰ مِن ثُلُثَىِ ٱلَّيۡلِ وَنِصۡفَهُ ۥ وَثُلُثَهُ ۥ وَطَآٮِٕفَةٌ۬ مِّنَ ٱلَّذِينَ مَعَكَ‌ۚ وَٱللَّهُ يُقَدِّرُ ٱلَّيۡلَ وَٱلنَّہَارَ‌ۚ عَلِمَ أَن لَّن تُحۡصُوهُ فَتَابَ عَلَيۡكُمۡ‌ۖ فَٱقۡرَءُواْ مَا تَيَسَّرَ مِنَ ٱلۡقُرۡءَانِ‌ۚ عَلِمَ أَن سَيَكُونُ مِنكُم مَّرۡضَىٰ‌ۙ وَءَاخَرُونَ يَضۡرِبُونَ فِى ٱلۡأَرۡضِ يَبۡتَغُونَ مِن فَضۡلِ ٱللَّهِ‌ۙ وَءَاخَرُونَ يُقَـٰتِلُونَ فِى سَبِيلِ ٱللَّهِ‌ۖ فَٱقۡرَءُواْ مَا تَيَسَّرَ مِنۡهُ‌ۚ وَأَقِيمُواْ ٱلصَّلَوٰةَ وَءَاتُواْ ٱلزَّكَوٰةَ وَأَقۡرِضُواْ ٱللَّهَ قَرۡضًا حَسَنً۬ا‌ۚ وَمَا تُقَدِّمُواْ لِأَنفُسِكُم مِّنۡ خَيۡرٍ۬ تَجِدُوهُ عِندَ ٱللَّهِ هُوَ خَيۡرً۬ا وَأَعۡظَمَ أَجۡرً۬ا‌ۚ وَٱسۡتَغۡفِرُواْ ٱللَّهَ‌ۖ إِنَّ ٱللَّهَ غَفُورٌ۬ رَّحِيمُۢ (٢٠) 

Verily this is an Admonition: therefore whoso will, let him take a (straight) path to his Lord!  The Lord doth Know that thou standest forth (to prayer) nigh two-thirds of the night, or half the night, or a third of the night, and so doth a party of those with thee.  But Allah doth Appoint night and day in due measure.  He Knoweth that ye are unable to keep count thereof.  So He hath Turned to you (in Mercy): read ye, therefore, of the Qur’an as much as may be easy for you.  He Knoweth that there may be (some) among you in ill-health; others travelling through the land, seeking of Allah’s Bounty; yet others fighting in Allah’s Cause.  Read ye, therefore, as much of the Qur’an as may be easy (for you); and establish regular prayer and give regular charity; and loan to Allah a Beautiful Loan.  And whatever good ye send forth for your souls, ye shall find it in Allah’s Presence ― yea, better and greater, in Reward, and seek ye the Grace of Allah: for Allah is Oft-Forgiving, Most Merciful. (Surah al-Muzzammil:19-20) 

To pray tahajjud then, is to be enfolded in the Divine Attributes and find Nearness.  It is a Gift for the believers and an Invitation to the Divine Presence.