Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Fasting is a shield, as long as a person does not tear it." It was asked: "How would he tear it?" He said: "By lying or by backbiting."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains a narrator, al-Rabi' bin Badr, and he is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5012