Source, in full
This has been widely attributed to Franklin since the 1940s, but is not found in any of his works. The language is not Franklin's, nor that of his time. It does paraphrase a portion of something he wrote in 1732 under the name Alice Addertongue
Notes from Wikiquote
If I have never heard Ill of some Person, I always impute it to defective Intelligence; for there are none without their Faults, no, not one. If she be a Woman, I take the first Opportunity to let all her Acquaintance know I have heard that one of the handsomest or best Men in Town has said something in Praise either of her Beauty, her Wit, her Virtue, or her good Management. If you know any thing of Humane Nature, you perceive that this naturally introduces a Conversation turning upon all her Failings, past, present, and to come.