This question already has an answer here:
Gayatri mantra which is chanted at the end of Sri sukta, which upanishad or Veda contains it?
The Mantra is this:
ॐ म॒हा॒दे॒व्यै च॑ वि॒द्महे॑ विष्णुप॒त्नी च॑ धीमहि । तन्नो॑ लक्ष्मीः प्रचो॒दया॓त् ॥ -- Om mahadevyai cha vidmahe vishnupatni cha dhimahi tanno lakshmi prachodayath ||
