مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3134
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "The month is complete with twenty-nine days.”
Musnad Ahmad 3158lts isnad is saheeh]