D'varim (Deuteronomy) 10.20
Et-YHVH Eloheycha tira oto ta’avod uvo tidbak uvishmo tishavea.
You shall fear the LORD your God; him shall you serve; and to him shall you cleave, and by his name shall you swear.

D'varim (Deuteronomy) 11.22
For if you shall diligently keep all this mitzvah which I command you, to do it, to love YHVH your Elohey, to walk in all his ways, and to cleave to him;

D'varim 30.20
to love YHVH your Elohey, to obey his voice, and to cleave to him; for he is your life, and the length of your days; that you may dwell in the land which YHVH swore to your fathers, to Avraham, to Yitzchak, and to Ya`akov, to give them.

Yehoshua (Joshua) 22.5
Only take diligent heed to do the mitzvah and the law which Moshe the servant of YHVH commanded you, to love YHVH your Elohey, and to walk in all his ways, and to keep his mitzvot, and to cleave to him, and to serve him with all your heart and with all your soul.

Acts 11:23 Who, when he came, and had seen the grace of Elohey, was glad, and exhorted them all, that with purpose of heart they would cleave unto YHVH.

The reason we need to cling to Him is stated in D'varim 30.20. He is our life! He is chayim! Clinging to Him is related to loving Him and obeying Him.

Shalom v'brakhot v'simcha,
Chizkiyah Shlomo (Carl)