The Prophet ﷺ said: When any of you prays, he should not place his sandals on his right side or on his left so as to be on the right side of someone else, unless no one is at his left, but should place them between his feet. (Using translation from Abū Dāʾūd 654)
"إذا صلَّى أحدُكم فلا يضعْ نعْليه عنْ يمينهِ ولا عنْ يسارِه، فتكونَ عن يمينِ غيره إلَّا أنْ لَا يكونَ عن يسارِه أحدٌ، وليضعْها بين رجْلَيه".