Where does “-щи” come from?

-щи (Old Church Slavonic) comes from Proto-Slavic -ťi, from Proto-Slavic -kti.

Ancestry of “-щи”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ťi	Form of *-ti, used with root-stem verbs ending in...
2	Proto-Slavic	-kti	—