What is the authenticity of Ibn ‘Uthaymeen’s fatwa permitting the shortening of the ‘Isha prayer to two rak‘ahs behind a traveler who is praying Maghrib, then performing the tasleem after the second rak‘ah or waiting until the imam performs the tasleem?

Shaykh Ibn Uthaymeen ruled that it is permissible for a traveler to shorten the Isha prayer to two rak'ahs if he prays behind an Imam who is praying Maghrib. His opinion varied regarding whether the worshiper should make the tasleem upon finishing the tashahhud (definition) or wait for the Imam and make the tasleem with him.

In some fatwas, he mentioned not making the tasleem except with the Imam, while in others, he allowed the intention to pray alone (infaarad) and make the tasleem.

He permitted intending to separate (from the Imam) and making tasleem after two rak'ahs as a shortened Isha prayer, because the Maghrib prayer is not among the shortened prayers.

He mentioned that if they (travelers) enter with an Imam who is praying Maghrib and they have not prayed Isha, then it is obligatory for them to follow the Imam, and when he makes tasleem, they should perform the fourth rak'ah. It is also possible that following the Imam for the fourth rak'ah is not obligatory, and they can sit in the first tashahhud, wait for the Imam, and make tasleem with him.

He also mentioned that if a person joins someone praying Maghrib, while intending to pray Isha shortened, he has a choice: if he wishes, he can intend to pray alone during the tashahhud, complete (his prayer), and make tasleem. If he wishes, he can continue with his Imam, and when his Imam makes tasleem, he can perform one rak'ah.

In another answer, he mentioned that if they pray Maghrib with the Imam for three rak'ahs and he makes tasleem, they should stand up and perform one rak'ah. They should not make tasleem after two rak'ahs; rather, they should sit or complete (the prayer) with him, and when he makes tasleem, they perform one rak'ah.