The Prophet (may Allah's peace and blessings be upon him) led them in the Zhuhr prayer and continued after the first two Rak‘ahs without sitting for Tashahhud, and people stood with him. When he finished the prayer and people were waiting for his making Taslīm, he said Takbīr while sitting and prostrated twice before Taslīm, then he concluded his prayer with Taslīm.
Authentic hadithعن عبد الله بن بُحَيْنَةَ رضي الله عنه وكان من أصحاب النبي صلى الله عليه وسلم «أن النبي صلى الله عليه وسلم صلَّى بهم الظهر فقام في الركعتين الأُولَيَيْنِ، ولم يَجْلِسْ، فقام الناس معه، حتى إذا قضى الصلاة وانتظر الناس تسليمه: كَبَّرَ وهو جالس فسجد سجدتين قبل أن يُسَلِّمَ ثُمَّ سَلَّمَ».
‘Abdullāh ibn Buhaynah (may Allah be pleased with him), who was one of the Prophet's Companions, reported: The Prophet (may Allah's peace and blessings be upon him) led them in the Zhuhr prayer and continued after the first two Rak‘ahs without sitting for Tashahhud, and people stood with him. When he finished the prayer and people were waiting for his making Taslīm, he said Takbīr while sitting and prostrated twice before Taslīm, then he concluded his prayer with Taslīm.
Explanation
The Prophet (may Allah's peace and blessings be upon him) led his Companions in the Zhuhr prayer. After completing the first two Rak‘ahs, he stood without sitting for the first Tashahhud. The people behind him followed suit. When he performed the last two Rak‘ahs and people were expecting him to make Taslīm, he said ''Allah Akbar'' while sitting and performed two prostrations, which are the two prostrations of forgetfulness, and then he made Taslīm. Those extra prostrations made up for the missed Tashahhud.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).