Sayyiduna Abu Ubaydah bin al-Jarrah (may Allah be pleased with him) narrates: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Fasting is a shield, as long as you do not tear it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8314