With the instruction from [God’s] mouth,
Eliphaz is of course referring to the instructions that he himself has given to Job. Ever since his night vision (Job 4:12–17), Eliphaz believes himself to possess divine wisdom. (Later, when God does speak to Job, we will see how different his instruction is from that of Eliphaz.)
22 Receive instruction from his mouth, and lay up his words in your heart.