The Source for this principle is the Passage of "V'H'yyeetem N'keeyim [You shall be Clean]": Pesachim 13a
Rules
Choosing between preventing suspicion, and violating Gd's Honor by sinning: Niddah 13a
Where a community is committing a suspicious act, they are less likely to be suspected: Rosh HaShanah 24b
Performing acts which could look illegal, in an area where no one will observe: Shabbat 146b; Beitzah 9a
Preventing suspicion via public pronouncement explaining an acts permissibility: Menachot 40a
Preventing suspicion by writing an open letter: Menachot 40a-b
Preventing an act which could cause others to question someone's lineage: Niddah 13a
Preventing misunderstanding of the law by on-lookers: Keritot 24b
Cases in which a person might be suspected of impropriety
Praying behind a synagogue: Berachot 6b
Separating Dough-Tithe from dough which is technically exempt, because the King is to take it for his tithe: Pesachim 6a
Sitting on a marketplace bench on Shabbat: Pesachim 51a
Moving a ladder, which might appear to be preparation for fixing one's roof, on Yom Tov: Beitzah 9a-b
Eating the milk which is at the outer lip of a cow's stomach; the prepared form is called "Di'Aytera": Pesachim 51a
Suspicion of having idolatrous forms: Rosh HaShanah 24b
Leaving laundry out to dry on Shabbat: Shabbat 146b; Beitzah 9a
Hanging laundry to dry inside a Succah, where it will appear to be a canopy of invalid material: Succah 10b
Suspicion of having a tattoo: Makkot 21a
Pouring out liquids in a private area from which they could run into a public area: Eruvin 88b
Concern lest people think that someone converted to Judaism for financial gain: Bava Metzia 72a
Drinking fish blood, which is normally permissible, but without scales present to identify it as fish blood: Keritot 21b
Suspicion in Issues of sexual impropriety
Giving Charity to a woman in private: Chagigah 5a
Avoiding reliance on a Single witness for testimony that her husband died: Ketuvot 22b
Not entering a ruin alone: Berachot 3a-b
Having a widow keep a dog around: Bava Metzia 71a [see Tosafot "Lo"]
If a woman, other than one's wife, has one's phylacteries: Berachot 23a
A student who killed suicide in his shame when framed for sexual immorality: Berachot 23a
Suspicion in Financial Matters
Clothing of the Kohen who was to collect the Half-Shekel Donation: Yevamot 102b
Suspicion of a Kohen who assists in the granary, and may seem to be seeking Tithes: Kiddushin 6b
Suspicion of a Public Official who Oversees Charity if he exchanges the leftover perishable coins to himself: Pesachim 13a, Bava Metzia 38a
The Overseer of the Communal Kitchen shouldn't buy the leftover food himself: Pesachim 13a, Bava Metzia 38a
Buying the Charge one is guarding, in a case where he is supposed to sell it due to rapid depreciation: Pesachim 13a
Showing the gold tables [which were bought with donations] to those who ascended to Jerusalem for Holidays, to show that there was no cheating in use of their Offerings: Pesachim 57a
Dealing with a partner upfront when dissolving an investing/managing partnership: Tosafot Bava Metzia 69b "ki" Rav's care in arranging a business investment, lest it appear like prohibited ribbit, because of his status as an important person: Bava Metzia 73a
Requiring a pauper to use funds in the way stated by his benefactor, lest people suspect that the benefactor had not fulfilled his commitment: Bava Metzia 78b
Suspicion of wrongdoing which might lead others to also be lax
License for a person to eat kodashim when he says he is pure, even if others say he is impure - but only to eat them in private, because of Marit Ayin concerns: Keritot 12b [see Rashi there]
