Musnad Ahmad 30:1383

'Affan narrated to us, Wuhayb narrated to us, 'Abdullah ibn Tawus narrated to us, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "We are the last, the first on the Day of Resurrection, except that every nation was given the Book before us, and we were given it after them. So this is the day they differed about, and Allah guided us to it; so tomorrow is for the Jews, and the day after tomorrow is for the Christians." Then he remained silent. Then he said: "It is the right of Allah upon every Muslim that he performs ghusl (full ablution) every seven days, washing his head and his body."