Source : Webster's Revised Unabridged Dictionary (1913)

Truthful \Truth"ful\, a.
   Full of truth; veracious; reliable. -- {Truth"ful*ly}, adv.
   -- {Truth"ful*ness}, n.

Source : WordNet®

truthfully
     adv : with truth; "I told him truthfully that I had just returned
           from my vacation"; "he answered the question as
           truthfully as he could" [ant: {mendaciously}]