Yehi Khevod
Overview
Although we don't know when Yehi Kevod was written, it seems to be an early part of Pesukei DeZimra. The verses of Yehi Kevod are almost identical in siddurim of the Geonim and Rishonim which usually shows the קדמות of the tefillah. Also a לקט פסוקים which starts with the verse of Yehi Kevod is found also in minhag Eretz Yisrael from the time of the Geonim which suggests that there may be an earlier מקור משותף.
Moshe Weinfeld says that Yehi Kevod was the original Pesukei DeZimra since it's named פסוקי after the verses of Yehi Kevod which would mean that Yehi Kevod was written before the time of the Amoraim.
Tannaim and Amoraim
Yehi Kevod is not found in any of our ספרות Tannaim or Amoraim.
Moshe Weinfeld says that Yehi Kevod is early and was written even before the Amoraim time. He says that originally Yehi Kevod was the main part of Pesukei DeZimra and that's why it's called פסוקי דזמרה and not פרקי דזמרה since Yehi Kevod is made out of pesukim and not perakim like Ashrei and the הללוי-ה.
Geonim
In the time of the Geonim, Yehi Kevod was said both in Bavel and Eretz Yisrael:
- Eretz Yisrael - Yehi Kevod is mentioned in מסכת סופרים and in Genizah fragments1. The Genizah fragments have a little different verses then we have.
- Bavel - Yehi Kevod is mentioned in the siddurim of the Geonim of R. Amram Gaon and Rasag. Both of them have basically the same pesukim that we say as part of Yehi Kevod.
Since, Yehi Kevod is found both in נוסח Eretz Yisrael and Bavel it seems to show that Yehi Keovd was composed early. In addition, נוסח בבל is almost identical to our נוסח which usually also shows that the tefillah was written early.