‘Umar (may Allah be pleased with him) reported that the Messenger (may Allah's peace and blessings be upon him) said: "He who prays behind the Imām is not liable for any act of forgetfulness (during the prayer). If the Imām forgets, he and those praying behind are liable for it. But if one who prays behind does forget, he is not liable for it, and the Imām suffices him."
شرح الحديث :
This Hadīth indicates that if someone who prays behind an Imam does something out of forgetfulness, there is nothing on him. On the other hand, if the Imām forgets, it applies to him and those praying behind, and so they should all offer the prostration of forgetfulness. This Hadīth is not authentically attributed to the Prophet (may Allah's peace and blessings be upon him), but its meaning is supported by another authentic Hadīth that says: "The Imām is appointed to be followed."