'Fast' in the Bible

Only be very careful to observe the commandment and the law which Moses the servant of the Lord commanded you, to love the Lord your God and walk in all His ways and keep His commandments and hold fast to Him and serve Him with all your heart and with all your soul.”

Instead, you are to hold fast to the LORD your God, as you have done today,

"But if you do at all go back, and hold fast to the remnant of these nations, even these who remain among you, and make marriages with them, and go in to them, and they to you;