Answered by Shaykh Amjad Rasheed
Is it permissible today to pray behind a Shi’i imam? If I have prayed behind him, do I have to make the prayer up?

It is valid to pray behind any Muslim who performs the prayer in its traditional manner by standing up, bowing, prostrating and sitting down, as well as fulfilling the prayer’s conditions such as facing the direction of prayer, clothing nakedness and performing purification.
However, it is offensive to pray behind an imam of the people of reprehensible innovations (ahl al-bida`).
If one does pray behind them, his prayer is valid and he does not have to make it up.