Mishkat al-Masabih 23 Medicine and Spells كتاب الطب والرقى 38

وَعَنْهُ مُرْسَلًا قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنِ احْتَجَمَ أَوِ اطَّلَى يَوْمَ السَّبْتِ أَوِ الْأَرْبِعَاءِ فَلَا يَلُومَنَّ إِلَّا نَفْسَهُ فِي الوَضَحِ» . رَوَاهُ فِي شرح السّنة

He reported in mursal form that God’s messenger said: “If anyone has himself cupped or smeared with something on a Saturday or a Wednesday, he must blame no one but himself if he gets leprosy.” It is transmitted in Sharh as-sunna .