Tafsir of Al-Ghashiyah 88:21

The fa in the words of the Exalted, "So remind" (fadhakkir), signifies the ordering of the command to remind upon what is indicated by the preceding rejection—namely, the lack of reflection. That is: restrict yourself to reminding, and do not be harsh with them, nor let it concern you that they do not reflect or take heed. And His words, "You are only a reminder," serve as the justification for the command.