ENOfficial
And Ibn ‘Abbas (rad) narrated in a Marfu’ (traceable) Hadith that: “Ablution is necessary for one who sleeps while he is lying flat. .
Source: sunnah.com
Other translations
Arabic
وَلِأَبِي دَاوُدَ أَيْضًا, عَنْ اِبْنِ عَبَّاسٍ مَرْفُوعًا: { إِنَّمَا اَلْوُضُوءُ عَلَى مَنْ نَامَ مُضْطَجِعًا } وَفِي إِسْنَادِهِ ضَعْفٌ أَيْضً ا 1 .
sunnah.com