التوبة

"There is no deity except You; exalted are You. Indeed, I have been of the wrongdoers." 

Surah al-Anbiyaa' [21:87]

"And seek forgiveness of Allah. Indeed, Allah is Forgiving and Merciful."  

Surah al-Muzammil [73:20]

"Alif Lam Ra. [This is] a Book whose verses are perfected an then presented in detail from [one who is] Wise and Acquainted. [Through a messenger, saying], "Do not worship except Allah. Indeed, I am to you from Him a warner and a bringer of good tidings," and [saying], "Seek forgiveness of your Lord and repent to Him, [and] He will let you enjoy a good provision."  

Suratul Hood [11:1-3]

O Believers! Repent to Allâh truly and sincerely - Surah Al-Tahrîm [66:8]

In explanation of this verse, Al-Tabarî reports the following narrations in his Tafsîr:

He reports that ‘Umar b. Al-Khattâb – Allâh be pleased with him – was asked about true repentance. He replied, “It is that a man repents from an evil deed, and never does it again.” He also reports that ‘Umar said, “It is that you repent from the sin and never do it again, or never intend to do it again.”