Musnad Ahmad 30:2345

Yahya ibn Sa'id al-Umawi narrated to us, he said: al-Hajjaj narrated to us, from 'Ata', from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fast upon seeing it, and break your fast upon seeing it. But if the month is obscured from you, then complete the count to thirty."