—except in the midst of the prayer, when a person who began as a lone worshipper changes his intention to that of an imam. According to the dominant opinion in the Hanbali school, this is not permissible unless he expects that a follower will arrive to join him, a ruling they draw from the occasion when Ibn ʿAbbās prayed with the Prophet ﷺ.
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It is permissible for someone offering a voluntary prayer to follow an imam who is performing an obligatory prayer, for the night vigil (tahajjud) was obligatory for him ﷺ while it is only recommended for his community. This is the well-known view of the school. As for one who is praying an obligatory prayer following an imam who is praying a voluntary prayer, two reports exist from Imām Aḥmad:
a) It is not permissible, and this is the well-known position of the school.
b) It is permissible, and this view has the stronger proof, based on an incident related in the two Ṣaḥīḥs. -
The ḥadīth also shows that a barrier between the imam and the congregation does not prevent the validity of the prayer or of following the imam. Al-Nawawī said: For following to be valid, the follower must know the imam’s movements—whether both are in the mosque, both outside, or one inside and the other outside—by unanimous agreement.
If one of them is outside the mosque yet can see the imam or the congregation, the prayer is valid even if the rows are not continuous, because the factor that would nullify is absent and the factor that establishes validity—sight of the imam and the ability to follow—is present.
Al-Insāf states that, on the sound view of the school, continuity of the rows is judged according to customary usage.
Al-Mughnī says: therefore no fixed measure is set, which is also the view of Mālik and al-Shāfiʿī, for no textual limit exists and it does not hinder following; what matters is whatever obstructs seeing the imam or hearing his voice. Al-Nawawī added that, outside the mosque, the distance must not become excessive, and this is the opinion of the majority of scholars.