Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: There is no ablution upon one who sleeps sitting with his knees drawn up (ihtiba'), nor upon one who sleeps standing, nor upon one who sleeps in prostration, until he lies down; once he lies down, he must perform ablution (this is a mawquf report).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 603