Among the proofs advanced by those who deem the prayer of an obligatory worshipper valid behind an imam who is praying voluntarily is the report that the Prophet ﷺ “led one group of his Companions in the prayer during times of danger (ṣalāt al-khawf) for two rakʿahs, pronounced the salām, then led the other group for another two rakʿahs and again pronounced the salām.” Abū Dāwūd relates this ḥadīth. In that second set, the Prophet was praying supererogatorily.

This situation does not fall under the forbidden type of contravening the imam, for what the ḥadīth forbids is failing to follow him in his movements—his standing, bowing, and prostration. After stating, “The imam has only been appointed to be followed,” the Prophet ﷺ continued: “When he pronounces the takbīr, you pronounce the takbīr; do not do so until he has done so,” and so on.

Among those who upheld this position is our teacher ʿAbd al-Raḥmān ibn Nāṣir al-Saʿdī (may Allah have mercy on him).

Points derived from the ḥadīth:

1. It is permissible for someone who is praying voluntarily to act as imam for worshippers performing an obligatory prayer; this is not the kind of contravention that is prohibited.
2. A fortiori, it is permissible for someone praying an obligatory prayer to lead worshippers whose prayer is voluntary.
3. It is permissible to repeat an obligatory prayer, especially when a benefit is realized—for example, when a person who can recite well leads others who cannot, or when someone enters the mosque after having prayed alone and finds a congregation; praying with them perfects what his solitary prayer had lacked.