Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Three things do not break the fast of a fasting person: vomiting, cupping and a wet dream."
This is how Abd al-Rahman bin Zayd narrated it, and he is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8273