Musannaf Ibn Abi Shayba 1:921

Ibn Fudayl narrated to us, from Yazid, from Mujahid, who said: While we were with Abdullah ibn Umar after he had prayed, he began rubbing his garment and said, "I searched for this last night and did not find it." Mujahid said: I do not see it as anything but semen.