I used to scrape it off the garment of the Messenger of Allah ﷺ with my hand. So if you see it, wash it, and if it is not visible to you, sprinkle water over it.
حدثنا محمد بن جعفر قال حدثنا سعيد عن أبي معشر عن النخعي عن الأسود بن يزيد
كنت أفركه من ثوب رسول الله ﷺ بيدي فإذا رأيته فاغسله فإن خفي عليك فارششه
Its isnad is sahih, its narrators are trustworthy and well-known. Saeed is Ibn Abu Aruba, Abu Mashar is Ziyad bin Kulayb, and Nakhai is Ibrahim bin Yazid. The hadith has previously appeared at 23924.