Musnad Ahmad 3492

It was narrated from ‘Urwah bin Murrah from Abul-Bakhtari who said: 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).”