It was not for nothing that the raven was just now croaking on my left hand.
Non temere est quod corvos cantat mihi nunc ab laeva manu
Original text (Latin)
Source, in full
Aulularia, line 624 (Act iv, sc. 3, 1); reported in Bartlett's Familiar Quotations, 10th ed. (1919). Referenced in "That raven on yon left-hand oak/(Curse on his ill-betiding croak!)/Bodes me no good", John Gay, Fables, Part I, The Farmer’s Wife and the Raven.