It is narrated from Abu al-Zinad that he said: I asked Sa'id bin al-Musayyib (may Allah have mercy on him) about a man who does not find (the means) to spend on his wife. He said: "They are to be separated." Abu al-Zinad said: I said: "Is this the Sunnah?" Sa'id (may Allah have mercy on him) said: "It is the Sunnah."
Imam al-Shafi'i (may Allah have mercy on him) said: "What Sa'id's saying ('Sunnah') resembles is that it is a Sunnah of the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15707