Sahih Bukhari Volume 002, Book 013, Hadith Number 025.
Narated By 'Aisha : (The wife of the Prophet) The people used to come from their abodes and from Al-'Awali (i.e. outskirts of Medina up to a distance of four miles or more from Medina). They used to pass through dust and used to be drenched with sweat and covered with dust; so sweat used to trickle from them. One of them came to Allah's Apostle who was in my house. The Prophet said to him, "I wish that you keep yourself clean on this day of yours (i.e. take a bath)."