I say, and with Allah's success: The ummah has unanimously agreed on the existence of the punishment of the grave, and they have differed on whether it applies to the soul and body or just the soul, and the first opinion is the relied-upon one. This is what the majority of the books of Sunni beliefs have agreed upon, and consensus is one of the strongest evidences in Sharia. It is based on the apparent meanings of many Quranic texts and the moral consensus in the hadiths. Therefore, whoever denies it is misguided but not a disbeliever, due to the abundance of evidence on this matter. The detailed evidence is widely available in books and on websites, so one can refer to it. One can respond to the person who denies it by saying that if he is Sunni, it is sufficient for him to know the agreement of the books of Sunni beliefs on it and the established consensus. If he is not Sunni, then the discussion should be about proving the methodology of the Ahl al-Sunnah regarding the punishment of the grave. And Allah knows best.