Sahih Muslim Book - 5, Hadith - 1287

وَحَدَّثَنِى الْقَاسِمُ بْنُ زَكَرِيَّاءَ حَدَّثَنَا حُسَيْنُ بْنُ عَلِىٍّ الْجُعْفِىُّ عَنْ زَائِدَةَ عَنْ سُلَيْمَانَ عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللَّهِ قَالَ صَلَّيْنَا مَعَ رَسُولِ اللَّهِ -صلى الله عليه وسلم- فَإِمَّا زَادَ أَوْ نَقَصَ - قَالَ إِبْرَاهِيمُ وَايْمُ اللَّهِ مَا جَاءَ ذَاكَ إِلاَّ مِنْ قِبَلِى - قَالَ فَقُلْنَا يَا رَسُولَ اللَّهِ أَحَدَثَ فِى الصَّلاَةِ شَىْءٌ فَقَالَ « لاَ ». قَالَ فَقُلْنَا لَهُ الَّذِى صَنَعَ فَقَالَ « إِذَا زَادَ الرَّجُلُ أَوْ نَقَصَ فَلْيَسْجُدْ سَجْدَتَيْنِ ». قَالَ ثُمَّ سَجَدَ سَجْدَتَيْنِ.

It was narrated that 'Abdullah said: "We offered prayers with the Messenger of Allah (s.a.w) and he either added or omitted something. We said: 'O Messenger of Allah, has something (new) been introduced into the prayer?' He said: 'No.' We told him what he had done and he said: 'If a man adds or omits something, let him prostrate twice.' Then he prostrated twice."