Is it permissible for a follower (Ma'mum) who started prayer behind an Imam to break away (separate) and complete the prayer individually due to a valid excuse?
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
It is permissible for a follower who intends to separate from the Imam and complete the prayer alone due to a valid excuse. Evidence for this is the narration of Jabir concerning Mu'adh who prayed with the Prophet (PBUH) and then led his people, reciting lengthy Surahs. A man fell behind and prayed alone. When confronted, he informed the Prophet (PBUH). The Prophet (PBUH) did not command the man to repeat the prayer nor did he object to his action. Valid excuses include hardship due to the Imam's lengthiness, illness, fear of sleepiness overcoming one, something that invalidates the prayer, fear of loss or damage to property, missing one's traveling companion, or the inability to find someone to stand next to if one leaves the row, and similar situations.
Supporting text
If the follower separates without a valid excuse, there are two narrations concerning the validity of the prayer. One narration states that the prayer is invalidated because leaving the Imam's following without excuse is akin to leaving it without the intention of separation. The second narration holds that the prayer is valid because if the follower intended to pray alone from the beginning, it would be valid according to one narration, thus the intention to pray alone subsequently is superior. Furthermore, a follower can become an individual by necessity when the Imam concludes the prayer early (the case of the one who misses parts of the prayer, Masbuq), whereas one cannot become a follower without intention under any circumstance.