مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3184
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Abul-Bakhtari said: We saw the new noon of Ramadan in Dhat ‘Irq, so we sent a man to Ibn 'Abbas (رضي الله عنه) to ask him. He [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.”
Musnad Ahmad 3208lts isnad is saheeh, Muslim (1088)]