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."

Hazrat Abu Al-Ala bin Shikhir narrates from his father that he offered prayer with the Messenger of Allah (peace be upon him), and the Prophet (peace be upon him) spat out phlegm and rubbed it with his left shoe. Khalid has added in his narration that "and the Prophet (peace be upon him) was in a region with hard ground." Imam Abu Bakr (may Allah have mercy on him) says that Abu Al-Ala is Yazid bin Al-Shikhir and is the brother of Mutarrif. The narrators of the hadith have attributed him to his grandfather. Imam Abu Bakr (may Allah have mercy on him) says that this hadith was narrated by Hammad bin Salamah from Jariri, who said, «‏‏‏‏عن ابي العلاء عن مطرف عن ابيه» ‏‏‏‏

Hazrat Mutarrif narrates from his father that I saw the Messenger of Allah (peace be upon him) praying, and he spat under his left foot. Hazrat Alaa added that then he rubbed it in.