Musnad Ahmad 30:3054

Waki’ narrated to us, Ali ibn al-Mubarak narrated to us, from Yahya ibn Abi Kathir, 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 of Ramadan by fasting a day or two, except for a man who used to fast a fast, so let him fast it."