It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever performs ablution on Friday has done well, and whoever takes a ritual bath, then the ritual bath is better, and the ritual bath is Sunnah." Imam Abu Dawud (may Allah have mercy on him) did not mention the words "and the ritual bath is Sunnah".
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1414