´Abu 'Ubaidah said:` "I heard the Messenger of Allah say: 'Fasting is a shield, so long as you do not damage it."'
Explanation & Benefits
Hafiz Muhammad Ameen
In another narration, the word "backbiting" (ghibah) is mentioned, meaning that backbiting and other similar sins wound the fast to such an extent that it will not serve as protection from the Fire, just as a shield with holes is of no use in battle. Thus, the fast will only become a shield from the Fire of Hell when the fasting person has avoided sins during his fast; otherwise, it may be rendered void.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2235