It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the Imam says «سَمِعَاللهُلِمَنْحَمِدَهُ», you should say «اللهمَّرَبَّنَالَكَالْحَمْدُ»; for whoever's 'Ameen' coincides with the 'Ameen' of the angels, his sins are forgiven."
Hadith Referenceالسنن المأثورة / باب ما جاء في القراءة في الركوع والسجود / 166
Hadith Takhrijصحیح بخاری، الاذان، باب فضل اللهم ربنا لك الحمد، رقم: 796