Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1844
Abu Huraira reported that the Apostle of Allah (may peace be upon him) said. it is the right of Allah upon every Muslim that he should take a bath (at least) on one day (Friday) during the seven days (of the week) and he should wash his head and body.