The Messenger of Allah ﷺ said: “Journey to Allah in the morning, the evening and a part of the night…” (Bukhārī)
These three periods are times of journeying to Allah through performing acts of obedience. These times are mentioned in many verses, including:
وَاذْكُرِ اسْمَ رَبِّكَ بُكْرَةً وَأَصِيْلًا ، وَمِنَ اللَّيْلِ فَاسْجُدْ لَهُ وَسَبِّحْهُ لَيْلًا طَوِيْلًا
“Remember the Name of your Lord in the morning and the evening. And during some parts of the night, prostrate before Him, and glorify Him throughout the long night.” (76:25-26)
In the morning and the evening, one finds both obligatory and optional actions to do. The obligatory actions are praying Fajr and ‘Asr, and these are the best two prayers from the farḍ prayers. These two prayers are performed during ‘the two cool periods’ and whoever preserves these two prayers shall enter Paradise. (Bukhārī)
As for the optional deeds, then it is to remember Allah after Fajr prayer until sunrise and after ‘Asr until sunset. There are numerous verses and aḥādīth which state the virtues of doing this.
The third period is the duljah: journeying in the last part of the night. This alludes to performing deeds at the end of the night which is the time for asking forgiveness.
About this time, Allah says: “And those who seek forgiveness before dawn,” (3:17) and elsewhere: “And in the hours before dawn they would seek forgiveness.” (51:18)
This refers to the last part of the time of the Descent in which Allah fulfils the needs of those asking and grants forgiveness to those who beg Him for it.
The middle of the night is reserved for the lovers who wish to spend time in seclusion with their Beloved (Allah) and the end of the night is reserved for the sinners to seek forgiveness for their sins.
Whoever is unable to join the striving of the lovers in the depth of the night should at least join the sinners pleading forgiveness at the end of the night.
‘I cannot imagine that someone would sleep during the last part of the night!’ (Ṭāwūs raḥimahullāh)
In Paradise, the first two periods (the morning and the evening) will be reserved for the elite of Paradise to see Allah. It is hoped that the one who preserves the two prayers (Fajr and ‘Asr) will be amongst these, more so if one continuously observes the adhkār and other forms of worship until sunrise and sunset.
If the servant adds to that, a journey in the last part of the night, then he has journeyed in all three times. And if he is true and sincere in the pursuit of this, it is hoped that he will achieve the greatest objective:
إِنَّ الْمُتَّقِيْنَ فِي جَنَّاتٍ وَنَهَرٍ ، فِيْ مَقْعَدِ صِدْقٍ عِنْدَ مَلِيْكٍ مُقْتَدِر
“Verily the righteous will be in the midst of gardens and rivers, on seats of truth in the presence of an All-Powerful Sovereign.” (54:55)
(Adapted from ‘al-Maḥajjah fī Sayr al-Duljah’ [The Journey to Allah] by Ibn Rajab raḥimahullāh)