The promise God made to Asa is found in
2 Chronicles 15:2:
"The Lord is with you while you are with him. If you seek him, he will be found by you, but if you forsake him, he will forsake you."
In essence, God promised his presence and favor as long as Asa and the people of Judah remained faithful to him and sought him. Conversely, God warned that he would withdraw his support if they turned away from him.