Joining a praying individual and thereby performing the congregational prayer
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Question: I went to the masjid and found the congregational prayer had finished and found someone praying by himself; Is it permissible for me to stand beside him and pray (along with him) on the basis that this person is the imaam and I am following (him)?

Response: Yes, it is permissible based upon the correct (opinion) of the two opinions of the scholars; If you find a person praying on his own and you have missed the congregational prayer, then there is no harm in you joining with him and (thereby) you both pray in congregation, since the congregational prayer is obligatory as much as one is able. The evidence for this is that the Prophet (sal-Allaahu `alayhe wa sallam) stood for the night prayer and then Ibn 'Abbaas (radhi-yAllaahu 'anhumaa) entered and joined him in the prayer, so he stood to the left of him and the Prophet (sal-Allaahu `alayhe wa sallam) moved him around to the right of him, (and this was) whilst the Prophet (sal-Allaahu `alayhe wa sallam) had (already) begun the prayer on his own and Ibn 'Abbaas (later) joined him and he (the Prophet) acknowledged this [Saheeh Muslim (1/525/526) from the hadeeth of Ibn 'Abbaas (radhi-yAllaahu 'anhumaa)]. So that which is permissible in the voluntary (prayers) is permissible in the obligatory (prayers) unless there is evidence to indicate otherwise.

Shaykh Saalih ibn Fowzaan
al-Muntaqaa min Fataawa Shaykh Saalih al-Fowzaan - Volume 5, Page 50, Fatwa No.75