וּמִֽמְּנַשֶּׁ֞ה נָפְל֣וּ עַל־דָּוִ֗יד בְּבֹא֨וֹ עִם־פְּלִשְׁתִּ֧ים עַל־שָׁא֛וּל לַמִּלְחָמָ֖ה וְלֹ֣א עֲזָרֻ֑ם כִּ֣י בְעֵצָ֗ה שִׁלְּחֻ֜הוּ סַרְנֵ֤י פְלִשְׁתִּים֙ לֵאמֹ֔ר בְּרָאשֵׁ֕ינוּ יִפּ֖וֹל אֶל־אֲדֹנָ֥יו שָׁאֽוּל׃
Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle, but athey helped them not; for the lords of the Philistines upon advisement sent him away, saying: ‘He will fall away to his master Saul to the jeopardy of our heads.’
Read midrashic interpretation of I Chronicles 12:20: and classical rabbinic exegesis.