ما حكم من قال إن الرسول صلى الله عليه وسلم حي، وهل تجوز الصلاة وراءه؟

If the speaker means that the Prophet (peace and blessings be upon him) is alive in his grave with a barzakh-like life (life in the interspace) that is more complete than the life of the martyrs, then this is true. However, if he means that he is alive like the life of this world, hearing, seeing, and providing succor, then this is false and contrary to Islamic law, because the Prophet (peace and blessings be upon him) died. The prayer behind such a speaker is valid, but he should be advised.