It is narrated from Hazrat Ahmar bin Juz’ that when the Prophet (peace be upon him) would go into prostration, we would feel compassion for the Prophet (peace be upon him) at that time because he would exert great effort in separating his elbows from his sides.
Hadith Referenceمسند احمد / أول مسند البصريين / 20337
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن فى المتابعات والشواهد لأجل عباد بن راشد
Narrated to us by Affan, narrated to us by Abbad ibn Rashid, narrated to us by al-Hasan, narrated to me by Ahmar, companion of the Messenger of Allah, peace and blessings be upon him, who said: "We used to see that the Messenger of Allah, peace and blessings be upon him, would keep his hands away from his sides when he prostrated."
Hadith Referenceمسند احمد / أول مسند البصريين / 20338
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن فى المتابعات والشواهد لأجل عباد بن راشد