The word(s) "unheard" appears 3 time(s) in 3 verse(s) in Quran in Abdullah Yusuf Ali translation.

(1) Then they proceeded: until, when they met a young man, he slew him. Moses said: "Hast thou slain an innocent person who had slain none? Truly a foul (unheard of) thing hast thou done!"

(سورة الكهف, Al-Kahf, Chapter #18, Verse #74)
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(2) He said: "Whoever doth wrong, him shall we punish; then shall he be sent back to his Lord; and He will punish him with a punishment unheard-of (before).

(سورة الكهف, Al-Kahf, Chapter #18, Verse #87)
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(3) Nor repulse the petitioner (unheard);

(سورة الضحى, Ad-Dhuhaa, Chapter #93, Verse #10)
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