﴿ قَالَ أَبَشَّرْتُمُونِى عَلَىٰٓ أَن مَّسَّنِىَ ٱلْكِبَرُ فَبِمَ تُبَشِّرُونَ ﴾

“Said he: "Do you give me this glad tiding despite the fact that old age has overtaken me? Of what [strange thing], then, are you giving me a tiding!"”

He said ‘Do you give me good tidings of a boy when old age has befallen me?’ ‘alā an massaniya l-kibaru is a circumstantial qualifier in other words despite the fact of its having befallen me? So of what thing do you give me good tidings?’ an interrogative meant to indicate amazement.