مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3063
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that the freed slave of Banu Nawfal - i.e., Abul-Hasan - said: Ibn 'Abbas was asked about a slave who divorced his wife twice, then they were manumitted; can he marry her? He said: Yes. It was said: From whom? He said: The Messenger of Allah (ﷺ) issued a verdict (fatwa) to that effect.
Musnad Ahmad 3088Its isnad is da'eef]