Musannaf Ibn Abi Shayba 3:5089

Abu Usama narrated to us, from Abd al-Rahman ibn Yazid, from Makhul, that Ibn Umar spat or expectorated in the mosque, and he forgot to cover it until he reached his house. Then he remembered after he had left, so he returned with a lamp and searched for it in the mosque until he covered it. Then he said: "Whoever spits in the mosque, it is a sin, and his repentance is to cover it."