Reference for I Samuel 20:18

וַיֹּֽאמֶר־ל֥וֹ יְהוֹנָתָ֖ן מָחָ֣ר חֹ֑דֶשׁ וְנִפְקַ֕דְתָּ כִּ֥י יִפָּקֵ֖ד מוֹשָׁבֶֽךָ׃

And Jonathan said unto him: ‘To-morrow is the new moon; and thou wilt be missed, thy seat will be empty.