Tafsir Al-Jalalayn: Al-Muminoon, Ayah 1

And from his narration on the authority of Ibn 'Abbas that he said regarding the meaning of Allah, Exalted is He, (Successful indeed are the believers): '(Successful indeed are the believers) He says: they are successful, safe and felicitous the ones who acknowledge Allah's divine Oneness. The latter are the ones who inherit Paradise, rather than the disbelievers; it is also said that this means: the believers will be safe due to their faith, and felicity is manifested in two ways: success and permanence.