Sayyiduna Sufyan bin Asid al-Hadrami (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "It is the greatest treachery that you tell your brother something in which he believes you, while you are lying to him in it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20846