Hadith: Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) performed one of the afternoon prayers (the sub-narrator Muhammad said: "I think that it was most probably the ‘Asr prayer") and he finished it after offering two Rak‘ahs only. He then stood near a piece of wood in front of the mosque and put his hand on it. Abu Bakr and ‘Umar were amongst those who were present, but they dared not talk to him about it (because of being awed by him), and those who were in a hurry went out. They said: "Has the prayer been reduced?" A man who was called Dhu al-Yadayn by the Prophet (may Allah's peace and blessings be upon him) said (to the Prophet): "Has the prayer been reduced or have you forgotten?" He said: "Neither have I forgotten, nor has the prayer been reduced." He said: "Certainly you have forgotten." So the Prophet offered two more Rak‘ahs and ended the prayer with Taslīm. Then he said Takbīr and performed a prostration like his ordinary prostration or a bit longer. Then he raised his head and said Takbīr. Then he put his head down and performed a prostration like his ordinary prostration or a bit longer. Then he raised his head and said Takbīr.' [Al-Bukhāri]

عن أبي هريرة رضي الله عنه قال: صَلَّى النبي صلى الله عليه وسلم إِحْدَى صَلاَتَيِ العَشِيّ -قال محمد: وَأَكْثَرُ ظَنِّي العصر- رَكْعَتَيْنِ، ثُمَّ سَلَّمَ، ثم قام إلى خَشَبَةٍ فِي مُقَدَّمِ المَسْجِدِ، فَوَضَعَ يَدَهُ عَلَيْهَا، وفيهم أبو بكر، وعمر رضي الله عنهما ، فَهَابَا أَنْ يُكَلِّمَاهُ، وخرج سَرَعَانُ النَّاسِ فَقَالُوا: أَقَصُرَتِ الصلاة؟ وَرَجُلٌ يَدْعُوهُ النبي صلى الله عليه وسلم ذُو اليَدَيْنِ، فَقَالَ: أَنَسِيتَ أَمْ قَصُرَتْ؟ فَقَالَ: لَمْ أَنْسَ وَلَمْ تُقْصَرْ، قَالَ: «بَلَى قَدْ نَسِيتَ، فَصَلَّى رَكْعَتَيْنِ، ثُمَّ سَلَّمَ، ثُمَّ كَبَّرَ، فَسَجَدَ مِثْلَ سُجُودِهِ أَوْ أَطْوَلَ، ثُمَّ رَفَعَ رَأْسَهُ، فَكَبَّرَ، ثُمَّ وَضَعَ رَأْسَهُ، فكبر، فَسَجَدَ مِثْلَ سُجُودِهِ أَوْ أَطْوَلَ، ثُمَّ رَفَعَ رَأْسَهُ وَكَبَّرَ».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) performed one of the afternoon prayers (the sub-narrator Muhammad said: "I think that it was most probably the ‘Asr prayer") and he finished it after offering two Rak‘ahs only. He then stood near a piece of wood in front of the mosque and put his hand on it. Abu Bakr and ‘Umar were amongst those who were present, but they dared not talk to him about it (because of being awed by him), and those who were in a hurry went out. They said: "Has the prayer been reduced?" A man who was called Dhu al-Yadayn by the Prophet (may Allah's peace and blessings be upon him) said (to the Prophet): "Has the prayer been reduced or have you forgotten?" He said: "Neither have I forgotten, nor has the prayer been reduced." He said: "Certainly you have forgotten." So the Prophet offered two more Rak‘ahs and ended the prayer with Taslīm. Then he said Takbīr and performed a prostration like his ordinary prostration or a bit longer. Then he raised his head and said Takbīr. Then he put his head down and performed a prostration like his ordinary prostration or a bit longer. Then he raised his head and said Takbīr. [Al-Bukhāri]
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

This noble Hadīth shows what to do when part of the prayer has been missed. One should complete what remains of the prayer, then perform Taslīm, and then perform two prostrations for forgetfulness. Abu Hurayrah reported that the Prophet led the Companions in the Zhuhr or ‘Asr prayer and finished after two Rak‘ahs. As the Prophet was perfect and only satisfied with perfect actions, he felt that there was a deficiency, without knowing the reason behind it. So he leaned up against a piece of wood in the mosque and intertwined his fingers. He was worried, because he felt that there was something incomplete. Some people left the mosque quickly through the doors, privately speaking among themselves that something had taken place, meaning that the prayer had been reduced. They found it hard to believe that it was possible for the Prophet to forget, because they were in awe of him. No one, not even Abu Bakr and ‘Umar dared to talk to him about the matter. However, a Companion named Dhu al-Yadayan broke the silence. He asked the Prophet: ''O Messenger of Allah, did you forget or has the prayer been reduced?'' The Prophet assumed that neither had happened. So learning that the prayer had not been reduced, Dhu al-Yadayn concluded that the Prophet had forgotten, because he was certain that he had only prayed two Rak‘ahs. The Prophet then realized that he had forgotten (part of the prayer). He wanted to make sure that Dhu al-Yadayn was correct, so he asked the Companions around him: ''Is it true what Dhu al-Yadayn is saying: that I only prayed two Rak‘ahs?'' They all said: ''Yes.'' So the Prophet moved forward and prayed the remaining two Rak‘ahs. After performing the Tashahhud, he performed the Taslīm. Then said Takbīr while he was sitting and prostrated as he would normally or a little longer and then raised his head from prostration. Next he said Takbīr and prostrated as he would normally or a little longer and then raised his head from prostration and said Takbīr. Finally, he performed Taslīm and ended the prayer.