What is the ruling on saying "Ameen" to the supplication of an innovator, whether in prayer or outside of it?

The general rule is that it is permissible to say "Amin" to the supplication (du'a') of a Muslim, as long as it does not contain a sin, even if the person is an innovator (mubtadi'), provided their innovation does not constitute disbelief (kufr). However, if their innovation constitutes disbelief, then there is a difference of opinion among the jurists regarding the permissibility of saying "Amin" to the supplication of a disbeliever. Some permitted it, while others prohibited it. Those who prohibited it cited the Almighty's saying: {And the supplication of the disbelievers is only in error.} Some scholars held the view that it is permissible, and even recommended, to say "Amin" if the disbeliever supplicates for guidance for themselves or for victory for the Muslims. They prohibited it if one is ignorant of what the disbeliever is supplicating for, as it might be a supplication involving sin.