Ibn Abbas (may Allah be pleased with them both) said in the explanation of this saying of Allah Almighty: "On that Day Allah will pay them in full their true recompense" [al-Nur: 25], that is, We will recompense them for their deeds with truth; just as He recompenses His friends with reward, so He recompenses His enemies with punishment. "And they will know" [al-Nur: 25], that is, on the Day of Resurrection, "that Allah is the Truth, the Manifest" [al-Nur: 25]. The reason for this is that Abdullah bin Ubayy bin Salul was in doubt about the religion and he was the chief of the hypocrites; what is meant is that the doubt left him and he became certain, at a time when that certainty could not benefit him.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19540