It is narrated from Sayyiduna Muawiyah (may Allah be pleased with him) that I heard the Messenger of Allah (peace and blessings be upon him): When the muezzin said «أَشْهَدُأَنْلَاإِلَهَإِلَّااللهُ», he (peace be upon him) said «أَشْهَدُأَنْلَاإِلَهَإِلَّااللهُ», and when the muezzin said «أَشْهَدُأَنَّمُحَمَّدًارَسُولُاللهِ», he (peace be upon him) said: "And I also bear witness to the Messengership," then he remained silent.
Hadith Referenceالسنن المأثورة / باب ما جاء في الصلاة على الراحلة / 127
Hadith Takhrijصحیح بخاری، الجمعة، باب يجيب الامام علی المنبر اذا سمع النداء، رقم: 914