Musnad Ahmad 26:433

Yahya ibn Adam narrated to us, Abu Bakr ibn Abdullah al-Nahshali narrated to us, he said: Abd al-Rahman ibn al-Aswad narrated to us, from his father, from Abdullah ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) prayed five [rak'ahs], either Dhuhr or Asr. When he finished, it was said to him: "O Messenger of Allah, has something been added to the prayer?" He said: "No." They said: "But you prayed five." He said: So he performed two prostrations of forgetfulness (sajdatay al-sahw), then he said: "I am only a human being; I remember as you remember, and I forget as you forget."