8:43
Remember in thy dream Allah showed them to thee as few: if He had shown them to thee as many, ye would surely have been discouraged, and ye would surely have disputed in (your) decision; but Allah saved (you): for He knoweth well the (secrets) of (all) hearts.
According to the commentary Anwar al-Bayan, before the Battle of Badr the Prophet Muhammad (peace be upon him) saw in a dream that the opposing forces were few in number. This vision encouraged the Companions and strengthened their resolve. The commentary explains that if God had shown the Prophet that the enemy forces were numerous, the believers might have become discouraged and begun to dispute among themselves about whether they should advance to fight or withdraw. The Qur’an refers to this event by stating that God showed the enemy as few in the Prophet’s dream so that the believers would not lose courage or fall into disagreement, emphasizing that God saved them from such hesitation because He knows what is hidden in people’s hearts (Qur’an 8:43). The commentary further explains that God is fully aware of those whose hearts are filled with love for Him and devotion to the Hereafter, as well as those who remain attached only to worldly life and lack concern for the Hereafter (Anwar al-Bayan, as cited in Islamic commentary).
References:
Madani, Muhammad. Anwar ul Bayan. (vol. 2 p. 393). Tafseer Anwar-ul-Bayan (English)