It is forbidden to spit on the right side while in Salat (prayers)

Narrated Abu Huraira and Abu Sa`id: Allah's Messenger (ﷺ) saw some expectoration on the wall of the mosque; he took gravel and scraped it off and said, "If anyone of you wanted to spit, he should neither spit in front of him nor on his right but could spit either on his left or under his left foot."