I've heard it's from
Aleksandr - Aleksasha -
Sasha. (The latter two are pet forms or hypocorisms).
Every language has its own suffixes for pet names. In russian it's -sha-:
Maria -
Masha,
Pavel -
Pasha,
Daria -
Dasha.
https://www.behindthename.com/glossary/view/diminutive