Musnad Ahmad 30:2579

Waki’ narrated to us, he said: Abu al-Umays ‘Utbah narrated to us, from al-‘Ala’ ibn ‘Abd al-Rahman ibn Ya’qub, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the middle of Sha’ban arrives, then refrain from fasting until Ramadan comes."