Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: When the Prophet (peace and blessings be upon him) passed away, he (peace and blessings be upon him) had nine wives, and he used to allot turns to eight.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14732