(a) It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he used to place his hands on the ground before his knees, and he would say that the Prophet (peace and blessings be upon him) used to do so.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2638