Musnad Ahmad 790:173

'Affan narrated to us, Hammad narrated to us, Thabit informed us, from Mutarrif, from 'Imran ibn Husayn, and Sa'id al-Jurayri, from Abu al-'Ala', from Mutarrif, from 'Imran ibn Husayn, that the Messenger of Allah (may Allah bless him and grant him peace) said to a man: "Have you fasted anything from the *surar* (end/secret parts) of Sha'ban?" He said: "No." He said: "Then when you break your fast for Ramadan, fast two days." Al-Jurayri said: "Fast one day."