Musnad Ahmad 689:1

Yahya ibn Zakariyya narrated to us, he said: Hajjaj informed us, from Husayn ibn al-Harith al-Jadali, who said: Abd al-Rahman ibn Zayd ibn al-Khattab delivered a sermon on the day about which there is doubt, and he said: "Behold, I have sat with the companions of the Messenger of Allah, may Allah bless him and grant him peace, and I asked them. Behold, they narrated to me that the Messenger of Allah, may Allah bless him and grant him peace, said: 'Fast at its sighting, and break the fast at its sighting, and perform the rites for it. If it is obscured from you, then complete thirty [days]. And if two Muslim witnesses testify, then fast and break the fast.'"