Numbers 15:28

And the priest shall make an atonement for the soul that sinneth ignorantly, when he sinneth by ignorance before the Lord, to make an atonement for him; and it shall be forgiven him.

Cross-references

In Leviticus 4:26, the priest makes atonement and forgiveness is granted — identical language to the atonement here.

Hebrews 10:26 contrasts willful sin with no sacrifice, directly opposing the provision here for unintentional sin.