Musannaf Ibn Abi Shayba 3:2117

Muhammad ibn Fudayl and Abdullah ibn Numayr narrated to us, from Yahya ibn Sa'id, from Abd al-Rahman al-A'raj, that Ibn Buhaynah informed him that the Messenger of Allah (may Allah bless him and grant him peace) stood up after two [rak'ahs] of the Dhuhr [prayer], having forgotten the sitting. Until when he finished his prayer, before he performed the taslim, he prostrated two prostrations of forgetfulness (sajdatay al-sahw) and performed the taslim.