(a) Amir (may Allah have mercy on him) narrates: I prayed behind Sayyiduna al-Nu'man bin Bashir (may Allah be pleased with him), and after two rak'ahs he began to rise. The people said 'Subhan Allah' (Glory be to Allah), so he sat down. Then, when he finished the prayer, he performed the two prostrations of forgetfulness, and we prostrated with him.
(b) In our view this is understood to mean that he had not stood fully upright. And it has been narrated to us from Yahya bin Sa'id (may Allah have mercy on him), from Sayyiduna Anas bin Malik (may Allah be pleased with him), that Sayyiduna Anas bin Malik (may Allah be pleased with him) began to stand up after two rak'ahs of the Asr prayer, so the people said 'Subhan Allah' (Glory be to Allah), and he sat down, then performed the two prostrations of forgetfulness while sitting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3845