It is narrated from Sayyiduna Hudhayfah bin al-Yaman (may Allah be pleased with him): I was sitting in the mosque of Madinah, my head nodding (from sleep), when a man embraced me from behind. I turned and saw that it was the Prophet (peace and blessings be upon him). I said: O Messenger of Allah, is ablution due upon me? He (peace and blessings be upon him) said: "No, not until you lay your side down."
Only Bahr bin Kunayz al-Saqqa narrated this hadith from Maymun al-Khayyat; he is da'if (weak), and his narration is not used as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 596