Musannaf Ibn Abi Shayba 3:5093

Waki' narrated to us, he said: Sayf ibn Sulayman al-Makki narrated to us, he said: I heard Mujahid say: Abu Ubaydah ibn al-Jarrah spat in the mosque at night, and he did not know where its location was. So he brought a lamp and searched for it until he covered it.