Hadith: If there was any change to the prayer, I would surely inform you; but I am a human being like you and liable to forget like you. So if I do forget, remind me, and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly, make Taslīm, and then offer two prostrations

عن عبد الله بن مسعود رضي الله عنه قال: صلى النبي صلى الله عليه وسلم -قال إبراهيم: لا أدري زاد أو نقص- فلما سلم قيل له: يا رسول الله، أحَدَثَ في الصلاة شيء؟ قال: «وما ذاك»، قالوا: صليتَ كذا وكذا، فَثَنَّى رِجليْهِ، واستقبل القبلة، وسَجَدَ سجدتين، ثم سلم، فلما أقبل علينا بوجهه، قال: «إنه لو حَدَثَ في الصلاة شيءٌ لنَبَّأَتُكُم به، ولكن إنما أنا بَشَرٌ مثلكم، أنسى كما تَنْسَوْن، فإذا نسَيِتُ فذَكِّرُوني، وإذا شَكَّ أحدكم في صلاته، فَلْيَتَحَرَّ الصَّوَابَ فليُتِمَّ عليه، ثم ليسلم، ثم يسجد سجدتين».
[صحيح] - [متفق عليه]

‘Abdullāh (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) offered the prayer. Ibrahīm, a sub-narrator, said: "I do not know whether he prayed more or less than usual." When he finished the prayer, he was asked: "O Messenger of Allah, has there been any change to the prayer?" He said: "What is it?" The people said: "You have prayed such-and-such." Thereupon, the Prophet (may Allah's peace and blessings be upon him) bent his legs, faced the Qiblah, performed two prostrations, and then made Taslīm. When he turned his face to us, he said: "If there was any change to the prayer, I would surely inform you; but I am a human being like you and liable to forget like you. So if I do forget, remind me, and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his prayer accordingly, make Taslīm, and then offer two prostrations."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

It is related in this noble Hadīth that the Prophet (may Allah's peace and blessings be upon him) led the Companions in prayer and prayed more or less than usual. So the Companions (may Allah be pleased with them) asked him whether any change had been made to the prayer. In response, he told them that if there were any change to the prayer, he would certainly keep them informed. Then he explained that he was only a human being like us and that he would forget like we do. After that, he presented the relevant ruling on a person who forgetfully adds something to prayer or subtracts something therefrom. He, the Prophet explained, should build upon the number of Rak‘ahs of which he is certain, complete his prayer accordingly, offer two prostrations of forgetfulness, and then make Taslīm, thus finishing the prayer.