Sayyiduna Anas (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) would go round to his wives with a single ritual bath (ghusl).
Ma'mar (may Allah have mercy on him) said: There is no doubt that he used to perform ablution (wudu) in between.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14086