Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.050A.

If one wakes from sleep and finds some moistness, but does not recall any wet dream, though he is sure it is sperm, what should he do? To be safe, he should perform ghusl. Said Mujahid and Qatadah, “There is no need for ghusl until he is sure that it is sperm, for his prior condition of purity is not ended by an uncertainty.