Musannaf Ibn Abi Shayba 3:98

Yunus ibn Muhammad narrated to us, he said: Yahya ibn Maymun narrated to me, he said: ‘Asim al-Ahwal narrated to me. He said: I saw Hafsa bint Sirin perform the takbir in prayer, and she gestured level with her breasts. And Yahya described it, so she raised both of her hands together.