Mustadrak al-Hakim — Hadith 1229

Abd al-Rahman bin Shumasah al-Mahri narrates: Sayyiduna Uqbah bin Amir al-Juhani (may Allah be pleased with him) led us in prayer, and he stood up (after the second rak'ah) instead of sitting. The people said: "Subhan Allah, Subhan Allah" (Glory be to Allah), but he did not sit and continued standing. Then, when he reached the last part of his prayer, he performed two prostrations of forgetfulness while sitting. After giving the salam he said: "I heard you just now saying 'Subhan Allah' so that I would sit down, but the fact is that the Sunnah is what I did."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.