Musnad Ahmad 30:3624

'Abd al-Samad and Abu 'Amir narrated to us, they said: Hisham narrated to us, from Yahya, from Abu Salamah, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Do not precede Ramadan by a day or two, unless it is a man who used to fast a fast, so let him fast it."