The Book of Mosques and Places of Prayer
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ إِسْمَاعِيلَ بْنِ أَبِي خَالِدٍ، عَنِ الزُّبَيْرِ بْنِ عَدِيٍّ، عَنْ مُصْعَبِ بْنِ سَعْدٍ، قَالَ رَكَعْتُ فَقُلْتُ بِيَدَىَّ هَكَذَا - يَعْنِي طَبَّقَ بِهِمَا وَوَضَعَهُمَا بَيْنَ فَخِذَيْهِ - فَقَالَ أَبِي قَدْ كُنَّا نَفْعَلُ هَذَا ثُمَّ أُمِرْنَا بِالرُّكَبِ .Ibn Sa'd reported:I bowed and my hands were in this state, i. e. they were put together, palm to palm, and were placed between his thighs. My father said: We used to do like this but were later on commanded to place them on the knees
From Sahih Muslim, whose reports the scholars accept as authentic.