Musnad Ahmad 30:2578

Waki' narrated to us, he said: Sufyan narrated to us, from Habib, from Ibn al-Mutawwas, from al-Mutawwas, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever breaks his fast for one day of Ramadan, without a concession (granted by Allah), the fasting of a lifetime will not suffice him."