Hadith: When the Prophet (may Allah's peace and blessings be upon him) prayed, he used to keep his upper arms away from his sides, such that the whiteness of his armpits would be visible

عن عبد الله بن مالك بن بُحَيْنَةَ رضي الله عنهم : «أن النبي صلى الله عليه وسلم كان إذا صَلَّى فرّج بين يديه، حتى يَبْدُوَ بياضُ إبْطَيْهِ».
[صحيح] - [متفق عليه]

‘Abdullāh ibn Mālik ibn Buhaynah (may Allah be pleased with him) reported: When the Prophet (may Allah's peace and blessings be upon him) prayed, he used to keep his upper arms away from his sides, such that the whiteness of his armpits would be visible.
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

When prostrating, and in order to lean on his hands and have them in a straight position, the Prophet (may Allah's peace and blessings be upon him) used to keep his upper arms away from his sides to the extent that the whiteness of his armpits would be visible. He used to do that because he was either leading people in prayer as the Imam or praying alone. As for the one praying behind the Imam, if such a posture would annoy the one praying next to him, then it will not be permissible for him to do that.