مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4272
Musnad of Abdullah bin Mas'ood
It was narrated that Ibn Mas'ood said: I did not fast Ramadan with the Prophet (ﷺ) with twenty-nine days more often that I fasted it with thirty.
Musnad Ahmad 4300Hasan because of corroborating evidence; its isnad is da'eef