مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2998
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that ‘Amr bin Murrah said: I heard Abul Bakhtari say: We saw the new moon of Ramadan when we were in Dhat ‘Irq, so we sent a man to Ibn ‘Abbas (رضي الله عنه) to ask him, Ibn ‘Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "Allah causes it to appear for long enough that people can see it, and if it is cloudy then complete the number (of days)."
Musnad Ahmad 3021Its isnad is saheeh, Muslim (1088)]