Teaching of the Prophet, narrated by Imam al-Baqir (A)

Al-Kulayni, in al-Kafi, reports with his chain of narrators from al-'Imam al Baqir (A) that he said, 

"The Messenger of Allah (S) said: 'God, the Sublime and the Blessed, said: 
"The workers (of deeds) for My sake should not, for My reward, trust the works they have done: for, verily, should they labour and toil for all their lives in My service and worship, they would fall short in their worship and will not attain to the reality of My service in what they seek with Me of My magnanimity and bounty, My paradise and the high stations in My proximity.

Rather, they should rely upon My mercy and put hope in My favour and be contented of their good opinion (husn al-zann) of Me; for, verily, therein My mercy will reach them, My good pleasure will go forth to them, and My forgiveness shall envelop them. For, indeed, I am Allah, the Beneficent and the Merciful, and I have been named so on that account"."
Al-Kafi, II, 71, hadith 1.