Hadith: I stayed overnight in the house of my aunt Maymūnah. When the Prophet (may Allah's peace and blessings be upon him) stood to perform Prayer at night, I joined him and stood on his left. He, then, took me by the head and made me stand on his right

عن عَبدُ اللَّهِ بنِ عَبَّاسٍ رضي الله عنهما قال: «بِتُّ عِندَ خَالَتِي مَيمُونَة، فَقَام النَبيَّ صلى الله عليه وسلم يُصَلِّي مِن اللَّيل، فَقُمتُ عَن يَسَارِه، فَأَخَذ بِرَأسِي فَأَقَامَنِي عن يَمِينِه».
[صحيح] - [متفق عليه]

Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported: I stayed overnight in the house of my aunt Maymūnah. When the Prophet (may Allah's peace and blessings be upon him) stood to perform Prayer at night, I joined him and stood on his left. He then took me by the head and made me stand on his right.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The great Companion Ibn ‘Abbās (may Allah be pleased with him) stated that he slept one night in the house of his maternal aunt, who is the Prophet's wife to learn about the Prophet's late-night prayer. When the Prophet (may Allah's peace and blessings be upon him) got up and stood to pray at night, Ibn ‘Abbās joined him in prayer and stood on the Prophet's left side to follow him in prayer. Since the right side is the most honorable of the two sides and the proper position where the one who prays behind an Imām should stand, in case he is one person, the Prophet (may Allah's peace and blessings be upon him) took him by the head, turned him from his behind and made him stand on his right.