Does the Bible say where there is unity there is strength?

Chalk it up to another colloquialism falsely attributed to scripture. No. The Bible does not say that. It may have been one of the founding fathers of the United States, but I am unsure who said it first.

Our forefathers and earlier Saints often used colloquialisms that were not actual Scriptural quotes, but were often Spiritual truths. For Instance "one can chase a thousand and two ten thousand" (Deuteronomy 32:30), also read (Ecclesiastes 4:9-12) and remember Jesus prayed that "Saints; we would be one, unified, so that the world might believe in Him" (John 17:21)

Although some expressions that we use are not actual quotes from Scripture, if we check them out carefully; we may discover that they do reflect spiritual truths that can be authenticated or verified by Scripture. Before you repeat it no matter how good it sounds, check it out.