Sayyidah Salma (may Allah be pleased with her), the wife of Sayyiduna Abu Rafi' (may Allah be pleased with him), narrates: I used to serve the Prophet (peace and blessings be upon him). Whenever the Prophet (peace and blessings be upon him) had a sore or a wound, he would instruct me to apply henna to it.
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطب / 8349