Sahih Al Bukhari Hadith Number 1225

Narrated `Abdullah bin Buhaina: Allah’s Messenger (peace eb upon him) got up after the second rak`a of the Zuhr prayer without sitting in between (the second and the third rak`at). When he finished the prayer he performed two prostrations (of Sahu) and then finished the prayer with Taslim.

22  Forgetfulness in Prayer
Hadith number 1225
Volume no 2
Chapter: 1 (What is said regarding the Sahw: If someone gets up (for the third rak’a without sitting) after the second rak’a of a compulsory salat (prayer)