St. Paul said to his disciple Timothy: "I remind you to rekindle the gift of God that is within you through the laying on of my hands" (2Tim 1:6), and "If any one aspires to the office of bishop, he desires a noble task." (1 Tim 3:1) To Titus he said: "This is why I left you in Crete, that you amend what was defective, and appoint presbyters in every town, as I directed you" (Titus 1:5).
Source: Catechism of the Catholic Church (Vatican.va)