Is a person's prayer invalid or without reward according to the Hanbalis if he prays behind an unrighteous person (fasiq) or an innovator (mubtadi') when it is possible to pray behind someone else, and what is the most preponderant opinion on this issue?

It is not permissible to pray behind an unrighteous person (fasiq) or an innovator (mubtadi') whose innovation is disbelieving (mukaffirah). As for an innovator whose innovation is not disbelieving and an unrighteous person, one should not pray behind them if someone else is available. However, prayer behind them is valid, though disliked (makruh (definition)). Whoever prays behind them when someone else is available has diminished the reward of their prayer. If refraining from praying behind them would cause one to miss the Friday prayer or the congregational prayer, then one should not refrain from praying behind them.