וַיִּשְׁמַ֥ע שָׁא֖וּל בְּק֣וֹל יְהוֹנָתָ֑ן וַיִּשָּׁבַ֣ע שָׁא֔וּל חַי־יְהוָ֖ה אִם־יוּמָֽת׃
And Saul hearkened unto the voice of Jonathan; and Saul swore: ‘As the LORD liveth, he shall not be put to death.’
Read kabbalistic (mystical) interpretation of I Samuel 19:6: and classical mystical sources.