"Ibn 'Abbas was asked about a slave who divorced his wife twice, then they were set free; could he marry her? He said: 'Yes.' He said: 'From whom (did you hear that)?' He said: 'The Messenger of Allah issued a Fatwa to that effect.'"
(One of the narrators) 'Abdur-Razzaq said: "Ibn Al-Mubarak said to Ma'mar: 'Which Al-Hasan is this? He has taken on a heavy burden.'"
Sunan an-Nasa'i Book 27, Hadith 40