What is the ruling on a person saying, "This soap has no effect now," while believing that black seed is a cure for diseases, and is this statement considered a sin?

If a person utters a statement that contradicts a prophetic hadith, while forgetting it or being unaware of it, then there is no sin upon him; because the Prophet, peace and blessings be upon him, said: "In the black seed there is healing for every disease except death."