“...O my Rabb! Expand me my breast. Ease My task for me; and remove the impediment from my speech, so they may understand what I say.”
(Qur‘an 20:25-28)

“Invite (all) to the Way of your Rabb with wisdom and beautiful preaching; and argue with them in ways that are best and most gracious: for your Rabb knows best, who have strayed from His Path, and who receive guidance.”  
(Qur‘an 16:125)

“Truly, Allâh will admit those who believe and work righteous deeds, to Gardens beneath which rivers flow. They shall be adorned therein with bracelets of gold and pearls; and their garments there will be of silk. For they have been guided (in this life) to the purest of speeches; they have been guided to the Path of Him Who is Worthy of (all) Praise.”
(Qur‘an 22:23-24)

“O my Rabb (Cherisher and Sustainer)! Bestow wisdom on me, and join me with the righteous; grant me honorable mention on the tongue of truth among the latest (generations); and make me one of the inheritors of the Garden of Bliss.” 
(Qur’an 26:83-85)

“Who is better in speech than one who calls (men) to Allâh, works righteousness, and says, `I am of those who bow in Islâm?’” 
(Qur‘an 41:33)

“O you who believe! Why say that which you do not? Grievously hateful is it in the sight of Allâh that you say that which you do not.” 
(Qur‘an 61: 2-3)

Abû Hurairah (r.a.) narrated that the Prophet (s.a.w.s.) said, “I have been given the keys of eloquent speech and given victory with awe (cast into the hearts of the enemy)...”
(Bukhâri 9/127)

Abdullah (r.a.) narrated that Allâh’s Messenger said, “The best talk (speech) is Allâh’s Book (Qur’ân) and the best way is the way of Muhammad. And the worst matters are the heresies (those new things which are introduced into the religion). And whatever you have been promised will surely come to pass, and you cannot escape (it).” 
(Bukhâri  9/382)

‘Abdullah Ibn Umar (r.a.) narrated that two men came from the East and addressed the people who wondered at their eloquent speeches. On that Allâh’s Messenger (s.a.w.s.) said, “Some eloquent speech is as effective as magic.” 
(Bukhâri  7/662)

Abu Hurairah (r.a.) narrated  that the Prophet (s.a.w.s.) said, “Whoever does not give up forged speech and evil actions, Allâh is  not in need of his leaving his food and drink (i.e. Allâh will not accept his fasting.)” 
(Bukhâri 3/127)

Malik (r.a.) narrated that the Prophet (s.a.w.s.) said, “Who believes in Allâh and the Last Day should talk what is good or keep quiet. (i.e., abstain from dirty and evil talk, and should think before uttering).” 
(Bukhâri  8/157)

Abu Hurairah (r.a.) narrated that the Prophet (s.a.w.s.) said: “On the Day of resurrection Allâh will not accept repentance or ransom from him who learns excellence of speech to captivate thereby the hearts of men, or of people.”  
(Abu Dawud 4988)