Is there any narration from the Shia Imams that says that you should pray your Maghrib prayer before breaking your fast?
Yes, it is recommended to pray Maghrib Prayer before breaking fast (Iftar) as Prayers is more important than breaking the fast, but it is not obligatory to do so. It is just recommended if there is no one waiting for you for Iftar and you do not feel weak if you pray before Iftar.
Wassalam.