It is narrated that the Prophet (pbuh) said: Modesty and restraint in speech are two affects of true faith, while foul talk and [adorned] rhetoric are two affects of hypocrisy.
Notes on the Text of the Narrative:
This narrative or a part of it, with minor variations, has been reported in Tirmidhiy (narrative no. 1950) Ahmad ibn Hanbal (narrative no. 21280), Ibn Abi Shaybah (narrative no. 30468), Abdul Razzaq (narrative no. 30468) Bayhaqiy (narrative no. 20597). The preferred text, except where otherwise mentioned, is the one reported in Ahmad ibn Hanbal (narrative no. 21280).
The same implication has also been conveyed in slightly different words, as follows:
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Hayaa (i.e. modesty) is a part of Iman and Iman shall be rewarded with Paradise. Foul talk is a part of neglect [towards God] and neglect [towards God] shall be rewarded with Hellfire.
This narrative has been reported in Ibn Maajah, narrative no. 4174.