Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) used to prostrate on the fleshy parts of the palm (the mounds at the base of the thumb).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2669