And Ibn ‘Abbas (rad) narrated in a Marfu’ (traceable) Hadith that:

“Ablution is necessary for one who sleeps while he is lying flat. [Reported by Abu Da’ud and there is weakness in its chain of narrators].

وَلِأَبِي دَاوُدَ أَيْضًا, عَنْ اِبْنِ عَبَّاسٍ مَرْفُوعًا: { إِنَّمَا اَلْوُضُوءُ عَلَى مَنْ نَامَ مُضْطَجِعًا } وَفِي إِسْنَادِهِ ضَعْفٌ أَيْضً ا [1]‏ .‏