It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him): "When the Messenger of Allah (peace and blessings be upon him) was in the first two rak'ahs, it was as though he were on heated stones." The narrator said: I asked: Until he stood up? He said: Until he stood up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2799
Another chain from Ibrahim bin Sa'd bin Ibrahim mentions it with the same meaning, and Shu'bah bin al-Hajjaj likewise narrated it from Sa'd bin Ibrahim.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2800