Musannaf Ibn Abi Shayba 1:862

Ghundar narrated to us, from Shu'bah, from Hammad, regarding a man who wakes up and sees wetness on his private part. He said: If he thinks that he had a wet dream, he should perform ghusl (full ablution). If he does not think that he had a wet dream, he should not perform ghusl. Qatadah said: If it was a spurting fluid, he should perform ghusl. I said to Qatadah: "How does he know?" He said: "By its smell." Al-Hakam said: He should not perform ghusl.