مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3715
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) used to teach us the tashahhud as he used to teach as a soorah of the Qur’an.
Musnad Ahmad 3738Saheeh, al-Bukhari (6265) and Muslim (402) this is a da'eef