Thanks, the sentence "Pray do me the favour to tell me why you were so melancholy, and wherefore you are no longer so." it is from the book The Arabian Nights. See all the paragraph below:
"I have found you immersed in a deep melancholy, and I have in vain attempted to remove it by different diversions. I imagined it might be occasioned by your distance from your dominions, or that love might have a great share in it; and that the queen of Samarcand, who, no doubt, is an accomplished beauty, might be the cause.
I do not know whether I am mistaken in my conjecture; but I must own, that it was for this very reason I would not importune you upon the subject, for fear of making you uneasy. But without myself contributing anything towards effecting the change, I find on my return that your mind is entirely delivered from the black vapour which disturbed it. Pray do me the favour to tell me why you were so melancholy, and wherefore you are no longer so."