The Albaani Site

Al-Albaani: The sunnah in the takbīr [of ʿEīd] differs from the prevailing sunnah in matters of adhkār, the general sunnah in matters of adhkār is for the dhikr to be done quietly and not out loud, but there are times where this sunnah is not the case and where saying it out loud is what is legislated: the takbīrs of ʿEīd are of this type … the takbirs of ʿEīd al-Aḍḥā are said out loud and not quietly.

Al-Hudā wan-Nūr, 618.