Ola-ika yu'tawna ajrahum marratayni bima sabaroo wayadraoona bilhasanati assayyi-ata wamimma razaqnahum yunfiqoon (surah Qasas: 54)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(28:54) These are the people who shall be given their reward twice *74 for the fortitude they showed; *75 they repel evil with good *76 and spend out of what We have provided them with. *77

Ulaaa`ika yu`tawna ajrahum marratayni bimaa sabaroo wa yadra`oona bil hasanatis saiyi`ata wa mimmmaa razaq naahum yunfiqoon

*74) That is, one reward for affirming faith in the Prophet Jesus (Allah's peace be upon him) and the second for affirming faith in Muhammad (may Allah's peace be upon him). The same thing has been expressed in the Hadith which Bukhari and Muslim have related on the authority of Hadrat Abu Musa Ash`ari. He says that the Holy Prophet said: "One of the three persons who will get a double reward is he who belonged to the people of the Book, had full faith in his Prophet and then affirmed faith in Muhammad (may Allah's peace be upon him)."
*75) That is, "They will get a double reward for the reason that they avoided nationalistic, racial and tribal prejudices and remained steadfast on the way of true Faith. When, on the advent of the new Prophet, they were confronted by a hard test, they proved by their conduct that they were not Christ-worshippers but God worshippers: they were not charmed by Christ's personality but were followers of "Islam". That is why when the new Prophet after Christ brought the same Islam that Christ had brought, they adopted the way of Islam under his leadership; without any hesitation, and gave up the way of those who remained stuck to Christianity. "
*76) That is, "They answer evil and falsehood with what is good and right: they repel injustice and mischief with what is just and noble; and they do not pay the other man back in the same coin. "
*77) That is, "They also make monetary sacrifices in the way of the truth. There might also be in it an allusion to this that those people had travelled from Habash to Makkah in search of the truth: they had no material gain in view when they undertook a toilsome journey involving a lot of expenses. When they heard that a man in Makkah had made a claim to be a Prophet, they thought it necessary to go and find out the truth, so that if a prophet had really been sent by God, they should not be deprived of affirming faith in him and being guided aright."