Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.058C.

If the imam makes the salah too long, it is permissible, under certain circumstances, to leave the salah with the intention of performing it individually. The following are examples of when this may be done: becoming ill, fearing that one’s wealth may be lost or destroyed, missing one’s companions or traveling group, being overcome by sleep, and so on. This is based on the following hadith related by Jabir: “Mu’azh would pray ‘isha with the Prophet sallallahu alehi wasallam, and then go and lead his people in salah. One night, the Prophet delayed salatul ‘isha and Mu’azh prayed with him and then went to his people and led them in the night prayer by reciting alBaqarah. One man left the salah and prayed by himself. The people said to him: ‘O so and so, you have become a hypocrite.’ He said: ‘I have not become a hypocrite but I shall surely go to the Prophet and inform him of what has happened.’ He told the Prophet what had happened and the Prophet said to Mu’azh: ‘You put people to trials, Mu’azh! You put people to trials, Mu’azh. Recite such and such surah.'” This is related by the group.