Tahajjud (Arabic: تَهَجُّد) means keeping vigil at night for performing night prayer, asking for forgiveness and reciting the Qur'an. According to verse 79 of Qur'an 17, night prayer was obligatory for the Prophet (s) and recommended for other believers. According to hadiths, tahajjud is an ornamentation in the hereafter, honor of believers, cause of health, expansion of daily sustenance, expiation for daily sins, contentment of God and adhering to the morals of prophets (a).
In usage, tahajjud refers to the night prayer, and mutahajjid is a person who gets up at night to perform prayer.
In the Qur'an
In the glorious Qur'an, the word "tahajjud" is mentioned once in "And keep vigil for a part of the night, as a supererogatory [devotion] for you."  According to this verse, night prayer was made obligatory for the noble Prophet (s) and he (s) received the position for making intercession on the Day of Judgment due to his night vigil and tahajjud. This verse addresses the noble Prophet (s), but, according to the verse, "There is certainly a good exemplar for you in the Apostle of Allah"  tahajjud has also been advised for the Islamic community. Also, from some hadiths, it can be deduced that night prayer was obligatory in early Islam and then it became a greatly recommended ritual.
Although the word tahajjud is mentioned once in the Qur'an, in different suras, night vigil and asking God for forgiveness at dawns are mentioned. Asking God for forgiveness at dawns is considered as one of the characteristics of the pious and keeping night vigil is regarded as one of the attributes of God's servants and true believers. Also, in verse 2 of the Qur'an 73, "Stand vigil through the night, except for a little [of it]," addresses the noble Prophet (s), but according to other verses and also hadiths, standing vigil through the night is advised for all.
The word tahajjud refers to the night prayer in hadiths too and has been advised, so much that it is transmitted from the Prophet (s) before his demise, in his wills to Imam Ali (a), that he (s) repeated three times, "I advise you to the night prayer."
According to hadiths, tahajjud is ornamentation in the hereafter (opposite to properties and children which are the ornamentations in this world), honor of believers, cause of health, expansion of daily sustenance, atonement for daily sins, contentment of God and adhering to the morals of prophets. In some hadiths, the person who does not practice tahajjud is criticized.
- Qur'an 17:79.
- Qur'an 33:21
- The material for this article is mainly taken from تهجد in Farsi WikiShia.