1 Samuel 20:13
The LORD do so and much more to Jonathan: but if it please my father to do thee evil, then I will shew it thee, and send thee away, that thou mayest go in peace: and the LORD be with thee, as he hath been with my father. The LORD do so and much more to Jonathan: but if it please my father to do thee evil, then I will shew it thee, and send thee away, that thou mayest go in peace: and the LORD be with thee, as he hath been with my father.
The promise belongs to the covenant that joined Jonathan and David as brothers after David struck down Goliath, at the time Saul retained David with himself and would not allow him to go back to his father's house. That covenant was grounded in Jonathan loving David as his own soul. The love was displayed when Jonathan stripped himself of his robe and handed over to David his garments together with his sword, his bow, and his girdle. At the same time David, though charged with important responsibilities, kept his modesty and gained the affection of the people and of the royal household.
In the verse Jonathan pledges that if his father purposes evil against David he will reveal it to him and send him away so that he may depart in peace, praying that the Lord be with David as He had been with Saul. Their parting confirms the covenant before the Lord and voices the desire for peace between them and between their descendants. Jonathan's sacred bond with David fulfilled God's purpose, for through Jonathan God kept David alive when Saul sought to kill him.