What is the ruling on a traveler performing the shortened prayer (Salat al-Qasr) alone in the mosque before the congregational prayer is established, and what is its ruling if he prays alone after the congregational prayer has been established while a congregation is present?

If a traveler enters a mosque and the prayer has not yet been established, it is better for him to wait and pray with the congregation. In this case, he should pray the first prayer in full, then shorten the second. It is permissible for a traveler to pray Maghrib behind an Imam who is praying Isha, or vice versa, provided he completes his prayer. It is better for the traveler to pray Maghrib alone, then pray Isha with them, to avoid the disagreement of those who stipulate that the intention of the Imam and the congregant must be the same. If he is unable to wait, there is no harm in him praying alone before the prayer is established or during it, due to an excuse. As for praying alone while the congregation is performing prayer without an excuse, it is valid, but the traveler incurs sin according to the view that congregational prayer is obligatory.