Ruth 2:15

And when she was risen up to glean, Boaz commanded his young men, saying, Let her glean even among the sheaves, and reproach her not:

Cross-references

Leviticus 19:9 commands leaving gleanings for the poor, the law behind Boaz's permission for Ruth to glean.

Leviticus 23:22 repeats the gleaning law, specifically including the sojourner like Ruth.