Abandon falsehood even when you think it will benefit youدعِ الْكَذِبَ حَيْثُ تَرَى أَنَّهُ يَنْفَعُكَ فإِنَّهُ يَضُرُّكَ،

, for it will ultimately harm you; and adhere to truthfulness even when you think it will harm you, for it will ultimately benefit you.
This proverb encourages making truthfulness a habit.
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This Arabic saying encourages honesty, even if it seems painful or ineffective, and avoidance of lying, even if it seems tempting or beneficial. A similar English proverb or saying is:
"Honesty is the best policy."
This is a well-known English proverb that encourages honesty.
Another saying that conveys a more complete meaning is:
"A lie may take care of the present, but it has no future."
Or:
"Tell the truth, even if it hurts; a lie may help now but harm later."
Similarly, Mark Twain said:
"If you tell the truth, you don't have to remember anything."
However, if you're looking for a common English proverb that corresponds to this Arabic saying, the best equivalent is:
"Honesty is the best policy."
Because it embodies the same idea: honesty is ultimately beneficial, and lying is harmful even if it seems beneficial at first.
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يضرب في الحث على لزوم الصدق حتى يصير عادة.