مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2362
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
'Ubaidullah bin ‘Abdullah bin 'Utbah bin Mas'ood narrated that 'Abdullah bin ‘Abbas told him: The Messenger of Allah (ﷺ) wrote to... And he narrated the same report.
Musnad Ahmad 2371Its isnad is saheeh, al-Bukhari (7) and Muslim (1173)]