Ikrimah (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning this saying of Allah Almighty: "And (forbidden to you are) married women, except those your right hands possess. (This is) the decree of Allah upon you." [Surah al-Nisa: 24] He said: It is not permissible for a Muslim to marry more than four; if he does, then she is (forbidden) to him like his mother and his sister.
Ubaydah al-Salmani (may Allah have mercy on him) said concerning this saying of Allah Almighty, "the decree of Allah upon you": It means four women. The same is narrated from al-Hasan al-Basri (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13848