Musannaf Ibn Abi Shayba 3:6158

Sahl ibn Yusuf narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about a man who arrives at a group of people while they are sitting, so he says the takbir (Allahu Akbar), then he sits. They both said: When he stands up, he counts that takbir.