28:54
Twice will they be given their reward, for that they have persevered, that they avert Evil with Good, and that they spend (in charity) out of what We have given them.
This passage is interpreted to describe the conduct of righteous individuals from Jewish or Christian backgrounds who later accept Islam. According to this understanding, such individuals exemplify the highest moral standards both before and after their conversion: they demonstrate sincerity and faithfulness within their original religious tradition and continue to embody these virtues after embracing Islam. Because they affirm both the earlier revelation and the message of Islam, they are described in Islamic teachings as receiving a double reward.
This concept is also reflected in a prophetic tradition in which the Prophet Muhammad stated that three types of people will receive their reward twice: a person from among the followers of earlier scriptures who believed in their prophet and later believed in Muhammad; a servant who fulfills his obligations faithfully both toward God and toward his employer; and a man who educates and treats his female servant with dignity, then frees her and subsequently marries her (al-Bukhārī).
References:
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.