Thabit bin Ubayd (may Allah have mercy on him) narrates: I prayed behind Mughirah bin Shu'bah (may Allah be pleased with him), and he did not sit in the second rak'ah, so the people said "Subhan Allah" to alert him, but he carried on with his prayer. When he had completed his prayer, he made the two prostrations of forgetfulness (sajdah al-sahw), then said the salam, then turned to the people and said: "Had you said Subhan Allah before I stood fully upright, I would have sat down; but the Messenger of Allah (peace and blessings be upon him) also led us in prayer in this very way."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18122