When his left side is not empty, and when he spits during prayer, it is also permissible to rub it with his foot.

Sayyiduna Tariq bin Abdullah Muharibi (may Allah be pleased with him) narrates that the Messenger of Allah (peace be upon him) said: "When you are in prayer, do not spit to your right side, but spit behind you or to your left side or under your left foot." This is the narration of Bundar. Abu Musa says that Mansur narrated to me and also said that the Messenger of Allah (peace be upon him) said to me: "Spit behind you, or spit to your left side if it is empty (there is no other worshipper), otherwise spit under your left foot in this manner."