Sahih Muslim Book - 5, Hadith - 1197

حَدَّثَنِى الْحَكَمُ بْنُ مُوسَى حَدَّثَنَا عِيسَى بْنُ يُونُسَ حَدَّثَنَا إِسْمَاعِيلُ بْنُ أَبِى خَالِدٍ عَنِ الزُّبَيْرِ بْنِ عَدِىٍّ عَنِ مُصْعَبِ بْنِ سَعْدِ بْنِ أَبِى وَقَّاصٍ قَالَ صَلَّيْتُ إِلَى جَنْبِ أَبِى فَلَمَّا رَكَعْتُ شَبَّكْتُ أَصَابِعِى وَجَعَلْتُهُمَا بَيْنَ رُكْبَتَىَّ فَضَرَبَ يَدَىَّ فَلَمَّا صَلَّى قَالَ قَدْ كُنَّا نَفْعَلُ هَذَا ثُمَّ أُمِرْنَا أَنْ نَرْفَعَ إِلَى الرُّكَبِ.

It was narrated that Mus'ab bin Sa'd bin Abi Waqqas said: "I prayed beside my father, and when I bowed, I interlaced my fingers and put them between my knees. He struck my hand and when he had completed his prayers, he said: 'We used to do that, then we were commanded to lift them to our knees."'