The source for “testing” Hashem with Ma’aser is an explicit pasuk in Tanach, from Malachi 10:3, which says (roughly translated), “Bring all the Ma’aser to the Bais HaOtzer…and test Me in this, if I will not open the windows of the Heavens (arubos hashamayim) and send you unbounded blessings.”
(Does anyone know the other two times “arubos hashamayim” are mentioned in Tanach? Extra credit if you can do this without a concordance.)