Forgetfulness in Prayer

Narrated Abu Huraira.:

Once Allah's Messenger (ﷺ) offered two rak`at and finished his prayer. So Dhul-Yadain asked him, "Has the prayer been reduced or have you forgotten?" Allah's Messenger (ﷺ) said, "Has Dhul-Yadain spoken the truth?" The people replied in the affirmative. Then Allah's Messenger (ﷺ) stood up and offered the remaining two rak`at and performed Taslim, and then said Takbir and performed two prostrations like his usual prostrations, or a bit longer, and then got up.