Sunan an-Nasai Book 12, Hadith 1032

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي يَعْفُورٍ، عَنْ مُصْعَبِ بْنِ سَعْدٍ، قَالَ صَلَّيْتُ إِلَى جَنْبِ أَبِي وَجَعَلْتُ يَدَىَّ بَيْنَ رُكْبَتَىَّ فَقَالَ لِي اضْرِبْ بِكَفَّيْكَ عَلَى رُكْبَتَيْكَ ‏.‏ قَالَ ثُمَّ فَعَلْتُ ذَلِكَ مَرَّةً أُخْرَى فَضَرَبَ يَدِي وَقَالَ إِنَّا قَدْ نُهِينَا عَنْ هَذَا وَأُمِرْنَا أَنْ نَضْرِبَ بِالأَكُفِّ عَلَى الرُّكَبِ ‏.‏

It was narrated that Mus'ab bin Sa'd said:"I prayed beside my father and I put my hands between my knees, and he told me: 'Put your hands on your knees.' Then I did that again and he struck my hands and said: 'We were forbidden to do that, and we were commanded to put our hands on our knees