Sahih Muslim 535 a
Mus'ab b. Sa'd reported:
I said prayer by the side of my father and placed my hands between my knees. My father said to me: Place your hands on your knees. I repeated that (the previous act) for the second time, and he struck at my hands and said: We have been forbidden to do so and have been commanded to place our palms on the knees.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَأَبُو كَامِلٍ الْجَحْدَرِيُّ - وَاللَّفْظُ لِقُتَيْبَةَ - قَالاَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي يَعْفُورٍ، عَنْ مُصْعَبِ بْنِ سَعْدٍ، قَالَ صَلَّيْتُ إِلَى جَنْبِ أَبِي قَالَ وَجَعَلْتُ يَدَىَّ بَيْنَ رُكْبَتَىَّ فَقَالَ لِي أَبِي اضْرِبْ بِكَفَّيْكَ عَلَى رُكْبَتَيْكَ ‏.‏ قَالَ ثُمَّ فَعَلْتُ ذَلِكَ مَرَّةً أُخْرَى فَضَرَبَ يَدَىَّ وَقَالَ إِنَّا نُهِينَا عَنْ هَذَا وَأُمِرْنَا أَنْ نَضْرِبَ بِالأَكُفِّ عَلَى الرُّكَبِ ‏.‏