(a) The Messenger of Allah (peace and blessings be upon him) forbade a man, when sitting in the prayer, to lean on his left hand.
(b) In the narration of Abu Dawud: The Messenger of Allah (peace and blessings be upon him) forbade sitting in the prayer while leaning on one's hand.
(c) This is the clearest of the narrations, and the narration of those other than Ibn Abd al-Malik does not contradict it, even though this one is clearer than them. The narration of Ibn Abd al-Malik is a mistake (wahm). What indicates that the narration of Imam Ahmad bin Hanbal (may Allah have mercy on him) is what is meant by the hadith is that Hisham bin Yusuf narrated it from Ma'mar in the same way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2809