What is the authenticity of using the hadith, "We used to wash it for the sick to be cured by it," narrated in Sahih Muslim, to deny Allah's exclusive power to heal the sick, in the context of calling people to Tawhid based on evidence from the Qur'an and Sunnah?

Muslim narrated from Asma, may Allah be pleased with her, that the Companions would wash the Prophet's ﷺ cloak for the sick, seeking healing through it. This does not mean that the Prophet ﷺ was the one who healed the sick, but rather that Allah placed healing and blessing in the Prophet's ﷺ relics as an honor to him. This is similar to how Allah placed healing in physical medicine.

The hadiths clarify that Allah alone is the Healer, as stated in the Prophet's ﷺ supplication for the sick: "Heal, for You are the Healer. There is no healing except Your healing." And as the boy said in the story of the People of the Ditch: "I do not heal anyone; only Allah heals." Therefore, healing is not attributed to anyone other than Allah.