What is meant by the statement of Ahmad ibn Hanbal: "A weak hadith is more beloved to me than opinion," and is it permissible to act upon a weak hadith?

The preponderant view is that a weak hadith is not to be used in rulings. It can, however, be used for the virtues of deeds, encouragement, and the like. Eminent scholars, such as Ibn Taymiyyah, have interpreted Imam Ahmad's statement that a weak hadith is more beloved to him than personal opinion, as meaning that he was referring to a hasan (good) hadith, not a weak, abandoned one. The terminology of hadith among early scholars was divided into sahih (authentic) and da'if (weak). The da'if was of two types: da'if matruk (weak and abandoned) and da'if ghayr matruk (weak but not abandoned), which is now known as hasan.