To put the hands (palms) on both knees while bowing.

Sahih al-Bukhari · Book 10 · Hadith 790

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.‏

Narrated Mus`ab bin Sa`d: 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…