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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.
Source: sunnah.com
Other translations
Arabic
وَعَن ابنِ عبَّاسٍ قَالَ: مِنَ السُّنَّةِ إِذَا جَلَسَ الرَّجُلُ أَنْ يَخْلَعَ نَعْلَيْهِ فَيَضَعَهُمَا بِجَنْبِهِ. رَوَاهُ أَبُو دَاوُدَ
sunnah.com