Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: When we touched someone suffering from leprosy, we would perform ablution afterwards.
Al-Tabarani recorded it in al-Mu'jam al-Awsat and al-Mu'jam al-Kabir; its chain contains a narrator, Jabir al-Ju'fi, whom Shu'bah and al-Thawri declared thiqah (trustworthy), while other people declared him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1275