Musnad Ahmad 30:3321

Muhammad ibn Abdullah al-Ansari narrated to us, Muhammad ibn Amr narrated to us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not precede the month—meaning Ramadan—by a day or two, unless that coincides with a fast that one of you used to observe. So fast upon seeing it, and break your fast upon seeing it. But if it is obscured from you, then complete thirty [days], then break your fast."