Sunan an Nasai
725
Sahih+2

It was narrated from Abu Sa'eed Al-Khudri that the Prophet (ﷺ) saw some spittle in the Qiblah of the Masjid. He scratched it off with a pebble and forbade a man to spit to his front or to his right. He said:"Let him spit to his left or beneath his left foot

SahihAbu Ghuddah
SahihAl-Albani