Mishkat al-Masabih 3 Purification كتاب الطهارة 34

وَعَنِ ابْنِ عَبَّاسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: ” إِنَّ الْوُضُوءَ عَلَى مَنْ نَامَ مُضْطَجِعًا فَإِنَّهُ إِذَا اضْطَجَعَ اسْتَرْخَتْ مفاصله. رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد

Ibn ‘Abbas reported God’s messenger as saying, “Ablution is necessary for one who sleeps lying down, for when he lies down his joints are relaxed.” Tirmidhi and Abu Dawud transmitted it.