مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4128
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah bin Mas'ood that The Messenger of Allah (ﷺ) would, after the two rak'ahs (i.e., in the first tashahhud), be as if he were (sitting) on baked stones. I (the narrator) said to Sa'd: Until he got up? He said: Until he got up.
Musnad Ahmad 4155lts isnad is da'eef because it is interrupted]