Sahur refers to the pre-dawn meal eaten during the time known as seḥer. The seḥer time is the period immediately preceding imsāk—that is, just before the beginning of the dawn prayer (ṣalāt al-fajr) and the commencement of the fast.
Rising for sahur is not a condition (sharṭ) for the validity of the fast; rather, it is a Sunnah. Therefore, if one does not partake in sahur, the fast remains legally valid (ṣaḥīḥ). However, one would not have acted in full accordance with the Prophetic practice and would be deprived of the reward associated with following the Sunnah. The Prophet (peace and blessings be upon him) himself observed sahur and recommended it to his community.
In a ḥadīth, the Messenger of Allah (peace and blessings be upon him) said:
“Partake in sahur, for indeed there is blessing (barakah) in sahur.”
(al-Bukhārī, Ṣawm, 20; Muslim, Ṣiyām, 45)
In another ḥadīth, he stated:
“Seek strength for the fast of the day with the pre-dawn meal (sahur), and for the night prayer (tahajjud) with a short midday rest (qaylūlah).”
(Ibn Mājah, Ṣiyām, 22)
The pre-dawn meal provides physical strength for fasting, thereby making the observance of the fast easier and more manageable.