Imam al-Shafi'i (may Allah have mercy on him) said: I do not know what to say about the land of al-Sawad (the cultivated land of Iraq) except a supposition joined to knowledge. That is because I found that the soundest hadith which the people of Kufah narrate about al-Sawad contains no clear statement, and I found among their hadiths others that contradict it: among them, they say al-Sawad was taken by treaty; they say al-Sawad was taken by force; and they say part of al-Sawad was taken by treaty and part of it by force.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18357