مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4048
Musnad of Abdullah bin Mas'ood
It was narrated from Abu 'Ubaidah that ‘Abdullah said: After the two rak’ahs (i.e., in the first tashahhud), the Prophet (ﷺ) would be as if he were (sitting on) baked stones.
Musnad Ahmad 4074Its isnad is da'eef because it is interrupted, Abu ‘Ubaidah