It is narrated from Sayyiduna Abu Shurayh (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The imam is a shield. If he completes the prayer, it is for you; and if he falls short, the burden of the shortfall is upon him, and the completeness is for you."
Hadith Referenceالمعجم الكبير للطبراني / باب الهاء / 18710