Musnad Ahmad 30:1454

'Affan narrated to us, Hammam narrated to us, Yahya ibn Abi Kathir narrated to us, Abu Salamah narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not precede Ramadan by fasting a day or two days, except for a man who has a habit of fasting, so let him fast."