Hadith: It is due upon every Muslim to bathe for one day every seven days, in which he should wash his head and body

عن أبي هريرة قال: قال رسول الله صلى الله عليه وسلم:
«حَقٌّ عَلَى كُلِّ مُسْلِمٍ أَنْ يَغْتَسِلَ فِي كُلِّ سَبْعَةِ أَيَّامٍ يَوْمًا، يَغْسِلُ فِيهِ رَأْسَهُ وَجَسَدَهُ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 897]

Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"It is due upon every Muslim to bathe for one day every seven days, in which he should wash his head and body."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 897]

Explanation

The Messenger of Allah (may Allah's peace and blessings be upon him) informed that it is due upon every rational and mature Muslim to take a bath once a week, in which he washes his head and body, seeking purification and cleanliness. The worthiest among these days is Friday, as understood from some versions. For example, bathing on Friday before the prayer is highly recommended, even if one has taken one on Thursday. What indicates that the bath on Friday is not obligatory is the statement by 'Ā’ishah (may Allah be pleased with her): "The people used to work, and whenever they went for the Friday prayer, they would go in the same condition as they had been in work. So, it was said to them: 'If only you had taken a bath'" [Narrated by Al-Bukhāri]. In another version by him: "They were smelling bad", i.e., the smell of sweat. Despite that, it was said to them: "If only you had taken a bath." so that others are more likely to do so.