Bulugh al-Maram 82
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 .‏‏1 ‏- منكر.‏ رواه أبو داود (202)‏.‏