Musnad Abu Ya'la al-Mawsili 117:157

Abu Bakr ibn Abi Shaybah narrated to us, Yazid ibn Harun narrated to us, Sadaqah ibn Musa narrated to us, Thabit al-Bunani narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about the best fasting. He said: "Sha'ban, in veneration of Ramadan."