Maliks Muwatta Book 3, Hadith Number 70.
Section : What to Do if One Stands After the Completion of the Prayer or After Two Rakas.
Yahya related to me from Malik from Ibn Shihab from al-Araj that Abdullah ibn Buhayna said, “The Messenger of Allah, may Allah bless him and grant him peace, prayed two rakas with us and then stood without sitting and the people stood with him. When he had finished the prayer and we had seen him say the taslim, he said ‘Allah is greater’ and did two sajdas from the sitting position and then said the taslim again.”