It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would go to all of his wives in a single night with a single ritual bath (ghusl)."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4977
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[حم ق ٤] عن أنس. الروض النضير ٨٨، صحيح أبي داود ٢١٢.