The word(s) "serious" appears 4 time(s) in 4 verse(s) in Quran in Abdullah Yusuf Ali translation.

(1) Behold, ye received it on your tongues, and said out of your mouths things of which ye had no knowledge; and ye thought it to be a light matter, while it was most serious in the sight of Allah.

(2) And why did ye not, when ye heard it, say? - "It is not right of us to speak of this: Glory to Allah! this is a most serious slander!"

(3) Then, when (the son) reached (the age of) (serious) work with him, he said: "O my son! I see in vision that I offer thee in sacrifice: Now see what is thy view!" (The son) said: "O my father! Do as thou art commanded: thou will find me, if Allah so wills one practising Patience and Constancy!"

(4) The mutual rivalry for piling up (the good things of this world) diverts you (from the more serious things),