Musnad Ahmad 31:580

Waki’ narrated to us, he said: Hammad bin Salamah narrated to us, from Bishr bin Harb, from Abu Sa’id al-Khudri, who said: "The Prophet, peace and blessings of Allah be upon him, forbade *wisal* (continuous fasting without breaking it) in fasting." And this sister of mine practices *wisal*, and I forbid her from it.