we asked Ibn ‘Abbas about sitting on heels between the two prostrations. He said: It is the sunnah. We said: We look upon it as a pressure on the foot. He said: This is the sunnah of your Prophet ﷺ  

أَنَّهُ سَمِعَ طَاوُسًا يَقُولُ قُلْنَا لاِبْنِ عَبَّاسٍ فِي الإِقْعَاءِ عَلَى الْقَدَمَيْنِ فِي السُّجُودِ فَقَالَ هِيَ السُّنَّةُ قَالَ قُلْنَا إِنَّا لَنَرَاهُ جَفَاءً بِالرَّجُلِ فَقَالَ ابْنُ عَبَّاسٍ هِيَ سُنَّةُ نَبِيِّكَ ﷺ