It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that forty of the companions of the Negus (al-Najashi) came to the Prophet (peace and blessings be upon him) and took part with him (peace and blessings be upon him) in the Battle of Uhud. Several of them were wounded, but none of them was killed. When they saw the hardship of the Muslims, they said: "O Messenger of Allah! We are people of means; permit us to bring our wealth and support the Muslims with it." So Allah, the Mighty and Majestic, revealed this verse concerning them: (Those to whom We gave the Scripture before it, they believe in it) (to the end of the verse), (Those will be given their reward twice for what they patiently endured); and so Allah appointed a double reward for them. Ibn Abbas (may Allah be pleased with them both) said: (And they repel evil with good) refers to the spending with which they supported the Muslims, until this verse was revealed.
He said: "Then the People of the Scripture began to boast over the Muslims, until this verse was revealed. They said: O community of Muslims! Whoever of us believes in your Book has two rewards, and whoever does not believe in your Book has one reward like your rewards. So Allah Almighty revealed: (O you who believe, fear Allah and believe in His Messenger; He will give you a double portion of His mercy and make for you a light by which you will walk). And so Allah increased them (the Muslims) in light and forgiveness as well." And He said: (So that the People of the Scripture may know that they have no power over anything of the bounty of Allah).
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Ja'far bin Abi al-Mughirah except Ya'qub al-Qummi, and Ali bin Thabit is alone in it."