O you who are grateful to yourself, let the gratitude be to you. ......أيها الممتن على نفسك فليكن المن عليك

 Gratitude, favor, and kindness are said to one who is kind to himself: You have attracted the benefit to yourself by what you have done, so do not boast about it to others.

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 Among the closest equivalents are:

Don't pat yourself on the back.

That is, "Don't praise yourself excessively."

No need to boast about doing your duty.

Your reward is in the deed itself.

That is, "Your reward lies in the deed itself."

Virtue is its own reward.

This is the closest in meaning; that is, "Virtue has its own reward." Whoever does good, the good returns to them, so there is no need for boasting or pride.

If you want a formulation closer to the spirit of the Arabic proverb itself, it can be translated metaphorically as:

If your kindness benefits yourself, don't boast of it to others.

الامتنان، الإنعام والإحسان، يقال، لمن يحسن إلى نفسه، قد جذبت بما فعلت المنفعة إلى نفسك فلا تمن به على غيرك.