Musannaf Ibn Abi Shayba 3:2546

Waki' narrated to us, he said: Sufyan narrated to us, from Ibn Abi Najih, who said: I slept in the Sacred Mosque (al-Masjid al-Haram) and had a wet dream in it. So I asked Sa'id ibn Jubayr, and he said: "Go and perform ghusl (ritual bath)." Meaning, he did not forbid him.