Musannaf Ibn Abi Shayba 3:2108

Ibn Fudayl narrated to us, from Yahya ibn Sa'id, who said: Anas led us in prayer during a journey, and he prayed the Asr prayer with us as two rak'ahs. When he sat in the second [rak'ah], he forgot to perform the taslim, and he went to stand up, so we glorified Allah (said "SubhanAllah") to him. When he sat, he performed the taslim and prostrated the two prostrations of forgetfulness.