Musannaf Ibn Abi Shayba 3:2538

Jarir narrated to us, from Yazid, from Ata, who said: A man said to Ibn Abbas, "I slept in the Sacred Mosque (al-Masjid al-Haram) and I had a wet dream." He said, "As for you taking it as a place to spend the night or a place to take a midday nap, then no. But as for you sleeping to rest or waiting for a need, then there is no harm."