His intellect betrayed him. دَلَّ عَلَيْهِ إِرْبُهُ.

دَلَّ عَلَيْهِ إِرْبُهُ. 
قال أبو عمرو: يقال للرجل الدميم تقتحمه العين ولا يُؤْبَنُ بشيء من النجدة والفضل: دل عليه إربه، أي عَقْله.
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Abu Amr said: It is said of an ugly man, one who is not easily recognizable and who possesses no virtue or merit: His intellect betrayed him.
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التعبير العربي «دَلَّ عَلَيْهِ إِرْبُهُ» يُقال للرجل الذي لا يلفت النظر بمظهره أو هيئته، لكن فضله وعقله وشجاعته تكشف عن قيمته الحقيقية.
ليس له مقابل حرفي في الإنجليزية، لكن من أقرب الأمثال والتعابير: Don't judge a book by its cover.
«لا تحكم على الكتاب من غلافه.»
أي لا تقوِّم الناس من مظهرهم، فقد تكون قيمتهم الحقيقية في عقولهم وفضائلهم.
ومن التعابير الأقرب إلى المعنى الخاص: His worth is known by his character, not his looks.
تُعرف قيمته بأخلاقه وصفاته، لا بمظهره.
Wisdom and merit speak for themselves.
الحكمة والفضل يدلان على صاحبهما.
Appearances can be deceiving.
المظاهر قد تكون خادعة.
وأقرب مثل إنجليزي شائع يؤدي المعنى العام هو:
Don't judge a book by its cover 

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The Arabic expression “دَلَّ عَلَيْهِ إِرْبُهُ” is used to describe a man whose appearance or demeanor doesn't immediately attract attention, but whose virtue, intellect, and courage reveal his true worth.
While there is no direct English equivalent, some of the closest proverbs and expressions include:
“Don't judge a book by its cover.”
“Do not judge a book by its cover.”
This means don't judge people by their appearance; their true value may lie in their intellect and virtues.
Other expressions that closely convey this specific meaning include:
“His worth is known by his character, not his looks.”
“Wisdom and merit speak for themselves.”
“Appearances can be deceiving.”
The closest common English proverb that conveys this general meaning is:
“Don't judge a book by its cover.”