It is narrated from Sayyiduna Sufyan bin Asad al-Hadrami (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "It is enough of a betrayal that you tell your brother something which he believes to be true, while you are lying in it."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6372