“(Muhammad), tell my servants who have committed injustice to themselves, ‘Do not despair of the mercy of Allah. Allah certainly forgives all sins. He is All-forgiving and All-merciful.’ (39:53)"
“…And do not despair of receiving comfort from Allah; only the unbelievers despair of receiving comfort from Him. (12:87)"
The Prophet (s) said: “On the Day of Resurrection, Allah will gather the despondent of His mercy with their faces blackened. They will be given the name of ‘the despondent of Allah’s mercy.’" ( 1 )
Amir ul-Mu'minin (a) said to the man who was prevailed by despair of God’s mercy out of his many sins: “You, to despair of the mercy of Allah is the greatest sin that you have ever committed." ( 2 )
1. Quoted from Safinat ul-Bihar; 2/451 (as quoted from ar-Rawandi’s an-Nawadir).
2. Quoted from Jami us-Sa’adat; part 1 page 246.
source : http://www.maaref-foundation.com