Musnad Ahmad 955:4

Abu al-Walid and ‘Affan narrated to us, they said: ‘Ubayd Allah ibn Iyad narrated to us, Iyad—meaning ibn Laqit—narrated to us, from Layla, the wife of Bashir, who said: I wanted to fast for two days continuously, but Bashir forbade me and said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade that and said: 'The Christians do that.'" ‘Affan said: "The Christians do that, but fast as Allah has commanded you, and complete the fast until the night. Then, when it is night, break your fast."