It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever spits towards his qiblah and does not cover it, it (the spittle) will come on the Day of Resurrection in the form of black marks, until it lands between his two eyes."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7929