Nafi' (may Allah have mercy on him) narrates that Abdullah bin Umar (may Allah be pleased with them both) said: "Whoever sleeps lying down, wudu becomes obligatory upon him, and whoever sleeps sitting, there is no wudu upon him."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 68
Hadith Takhrijقد صح هذا الأثر من فعل ابن عمر رضي الله عنهما: أخرجه عبد الرزاق: (484)، وابن أبي شيبة: (1402).