Musnad Ahmad 30:2526

Yahya narrated to us, from Muhammad, Abu Salamah narrated to us, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Do not precede the month by a day or two, unless it coincides with a fast that one of you used to observe. Fast upon seeing it, and break your fast upon seeing it. If it is obscured from you, then complete thirty days, then break your fast."