مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3451
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
‘Amr bin Dinar narrated that he heard Muhammad bin Jubair say: Ibn ‘Abbas (رضي الله عنه) used to object to starting the fast of Ramadan early if the new moon of Ramadan has not been seen. And he used to say: The Prophet (ﷺ) said: "If you do not see the new moon, complete thirty days."
Musnad Ahmad 3474Its isnad is da'eef]