It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the wives of the Prophet (peace and blessings be upon him) were in two groups: in one group were Aishah, Hafsah, Safiyyah and Sawdah (may Allah be pleased with them), and in the other group were Umm Salamah and the rest of the wives of the Prophet (peace and blessings be upon him) (may Allah be pleased with them).
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19436