Sincerity is mentioned in the Quran in these verses:
[2:265] The example of those who give their money seeking God’s pleasure, out of sincere conviction, is that of a garden on high fertile soil; when heavy rain falls, it gives twice as much crop. If heavy rain is not available, a drizzle will suffice. God is Seer of everything you do.
[10:22] He is the One who moves you across the land and sea. You get onto the ships, and they sail smoothly in a nice breeze. Then, while rejoicing therein, violent wind blows, and the waves surround them from every side. This is when they implore God, sincerely devoting their prayers to Him alone: “If You only save us this time, we will be eternally appreciative.”
[17:67] If you are afflicted in the middle of the sea, you forget your idols and sincerely implore Him alone. But as soon as He saves you to the shore, you revert. Indeed, the human being is unappreciative.
[30:38] Therefore, you shall give the relatives their rightful share (of charity), as well as the poor, and the traveling alien. This is better for those who sincerely seek God’s pleasure; they are the winners.
[31:32] When violent waves surround them, they implore God, sincerely devoting their prayers to Him alone. But as soon as He saves them to the shore, some of them revert. None discards our revelations except those who are betrayers, unappreciative.
[39:8] When the human being is afflicted, he implores his Lord, sincerely devoted to Him. But as soon as He blesses him, he forgets his previous imploring, sets up idols to rank with God and to divert others from His path. Say, “Enjoy your disbelief temporarily; you have incurred the hellfire.”
[49:17] They act as if they are doing you a favor by embracing Submission! Say, “You are not doing me any favors by embracing Submission. God is the One who is doing you a great favor by guiding you to the faith, if you are sincere.”
[56:77] This is an honorable Quran.
[56:78] In a protected book.
[56:79] None can grasp it except the sincere.
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