Mishkat al-Masabih 22 Clothing كتاب اللباس 106

وَعَن ابنِ عبَّاسٍ قَالَ: مِنَ السُّنَّةِ إِذَا جَلَسَ الرَّجُلُ أَنْ يَخْلَعَ نَعْلَيْهِ فَيَضَعَهُمَا بِجَنْبِهِ. رَوَاهُ أَبُو دَاوُدَ

Ibn Abbas said it is part of the sunna that when a man sits down he should take off his sandals and place them at his side. Abu Dawud transmitted it.