Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "We are the last (of all nations) and will be the foremost on the Day of Resurrection, even though every (nation) was given the Book before us and we were given it after them. This is the day about which they differed, and Allah guided us to it. For the Jews was the next day (Saturday), and for the Christians the day after (Sunday)." He (peace and blessings be upon him) fell silent and then said: "It is a right upon every Muslim to take a ritual bath once in every seven days, washing his head and his body."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1421