It is narrated from Muhammad ibn Ali ibn Mahbub, from Ahmad, from Ali ibn Mahbub, from Ahmad, from Ali ibn Saif, from his father, from al-Husayn ibn Khalid al-Sayrafi. He (Narrator) said: “I asked Abu al-Hasan al awwal (the First) عليه السلام, ‘Why has the ghusl (ritual purification) of Friday been made obligatory?’" Imam عليه السلام replied: "Indeed, Allah سبحانه وتعالى the Exalted has completed the obligatory prayer with the supererogatory prayer, and He سبحانه وتعالى completed the obligatory fasting with supererogatory fasting, and He سبحانه وتعالى completed the obligatory wudu (ablution) with the ghusl of Friday, to make up for any error, shortcoming, or forgetfulness therein."