Musannaf Ibn Abi Shayba 7:336

Ali bin Mushir narrated to us, from al-Shaybani, from Abd al-Rahman ibn al-Aswad, from his father, from Abu Hurayrah, who said: I was sitting with Ibn Mas'ud above his house, and the sun set. Abdullah said: "By the One besides Whom there is no god, this is when the fasting person breaks his fast."