Bulugh al-Maram, Book 1, Hadith 88 (English) ⓘ
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 (99). 2 - منكر. رواه أبو داود (202).
also referenced under: Book 1, Hadith 99 (In-Book)