salaams, sometimes when i goto mosque for isha namaz the tarawih is being prayed, should i pray my isha namaz seperatly first or can i join the tarawih with the intention of isha namaz and when the imam says assalumualaykum i get up and pray the other 2 rakats, ive heard from DR ZAKIR that i can join the imam then finish of the 2 rakats, how true is this?