Full Idea
A reason is a known truth whose connection with some less well-known truth leads us to give our assent to the latter.
Gist of Idea
A reason is a known truth which leads to assent to some further truth
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 4.17)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.475
A Reaction
This is plainly false, because you can have a reason for believing something, but still not give your assent to it, presumably because of counter-reasons. And a false belief could also be a reason, even to believe a truth. Tut tut.