Musannaf Ibn Abi Shayba 3:241

Ya'la narrated to us, from al-A'mash, from Ibrahim, from al-Aswad, who said: When Umar raised his head from the bowing (ruku'), he would say, "Allah hears those who praise Him" (Sami' Allahu liman hamidah) before he straightened his back. And when he said the takbir (Allahu Akbar), he would say the takbir while he was descending.