The verse comes from the New Testament book titled Philippians. The verse is found in the 4th chapter, 13th verse: "I can do all things through Christ who strengthens me." Depending on which translation you're reading from, the verse may read differently, such as: "For I can do everything through Christ, who gives me strength" or "I can do everything God asks me to with the help of Christ who gives me the strength and power."
The words of this verse were written by the Apostle Paul. Most theologians are in agreement that he wrote Romans; 1 and 2 Corinthians; Galatians; Philippians; 1 and 2 Thessalonians; Philemon; Ephesians; Colossians; 1 and 2 Timothy; and, Titus. These 13 “letters” (books) make up the “Pauline Authorship” and are the primary source of his theology. The Apostle Paul spent his life proclaiming the risen Christ Jesus throughout the Roman world, often at great personal peril.
Through the words of Philippians 4:13 (which Paul wrote while imprisoned), Paul attributed his spiritual muscle to its true source — Jesus Christ... "I can do all things through Christ who strengthens me." Paul rejoiced not in his own will and courage, but in the power of Christ in him. By that example, we know that we also have access to the same spiritual power and courage. Paul's words in Scripture reveal that whether we face adversity, persecution, affliction, hardship or disaster, God’s remarkable purpose for us is far greater and is not to be compared with this life’s struggles or sufferings. I know without a doubt that God has truly strengthened me to accomplish tasks and endure situations and circumstances that I honestly don't believe I could have done by my own willpower. I've learned to accept (and appreciate) that when I am weak, God is most strong in my life.