Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "There are three things that do not break the fast of a fasting person: vomiting, cupping and a wet dream."
Abd al-Rahman bin Zayd bin Aslam is da'if (weak), and what is mahfuz (preserved) from Zayd bin Aslam is the first narration.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8034