Bulugh al-Maram 753

Anas (RAA) narrated, ‘When we assumed Ihram (for Hajj or 'Umrah) some of us raised their voices with Talbiyah and nobody objected to that, and others raised their voices with Takbir and no one objected to that (which means that saying Allahu Akbar ‘Allah is the Greatest’, or reciting the Talbiyah are both acceptable during Ihram).’ Agreed upon.