(a) It is narrated from al-Hasan (may Allah have mercy on him) that whoever falls asleep while sitting should perform ablution.
(b) It is narrated from al-Hasan (may Allah have mercy on him) that whoever falls asleep sitting or standing must perform ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 589