Sahih al-Bukhari 790
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · To put the hands (palms) on both knees while bowing.
Narrated Mus`ab bin Sa`d:
حَدَّثَنَا أَبُو الْوَلِيدِ ، قَالَ حَدَّثَنَا شُعْبَةُ ، عَنْ أَبِي يَعْفُورٍ ، قَالَ سَمِعْتُ مُصْعَبَ بْنَ سَعْدٍ ، يَقُولُ صَلَّيْتُ إِلَى جَنْبِ أَبِي فَطَبَّقْتُ بَيْنَ كَفَّىَّ ثُمَّ وَضَعْتُهُمَا بَيْنَ فَخِذَىَّ، فَنَهَانِي أَبِي وَقَالَ كُنَّا نَفْعَلُهُ فَنُهِينَا عَنْهُ، وَأُمِرْنَا أَنْ نَضَعَ أَيْدِيَنَا عَلَى الرُّكَبِ.
I offered prayer beside my father and approximated both my hands and placed them in between the knees. My father told me not to do so and said, "We used to do the same but we were forbidden (by the Prophet) to do it and were ordered to place the hands on the knees."