The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, commented: "If a disbeliever shakes hands with a Muslim and his hand is wet with sweat or anything else, the Muslim should wash it with water. But if there is no moisture, he should wipe it on a wall or soil." This ruling is supported by the verse: "Indeed, the polytheists are impure." (Surah At-Tawbah 9:28) Since the verse explicitly declares their impurity, anything they touch is considered impure, except for cases specifically permitted by Shariah. This interpretation is further supported by other narrations and evidence.