Musnad Ahmad 30:3531

Narrated to us Rawh, narrated to us Hisham bin Abi Abdullah and Husayn bin Dhakwan, from Yahya bin Abi Kathir, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not precede Ramadan by fasting a day or two, except for a man who used to fast a fast, so let him join it with it."