It is narrated from Ahmad ibn al-Hasan, from his father, from Muhammad ibn Marwan ibn Muslim, from Muhammad ibn Abdullah, from Abu Abdullah عليه السلام. Imam عليه السلام said: "The Ansar used to work on their fields and properties. When Friday came, people would be disturbed by the odours from their armpits and bodies. So the Messenger of Allah سبحانه وتعالى (peace be upon him and his family) commanded them to perform ghusl (ritual purification) on Fridays, and thus it became a Sunnah (established practice)."