Muammad b. āli Al-`Uthaymīn has objected to taking proof from the aforementioned verse of the Qur'ān  [and had they, when they were unjust to themselves, come to you and asked forgiveness of Allah and the Messenger had (also) asked forgiveness for them, they would have found Allah Oft-returning (to mercy), Merciful] as he said in his formal legal opinions (Fatāwā [1]) what follows:  

"Idh (when) is an adverb [denoting time] that has passed by and not an adverb [denoting action that will take place] in the future. Allāh has not said: 'And had they, when they are unjust', rather Allāh said 'when (idh) they were unjust'. Hence this verse is referring to something that took place during the life of the Messenger of God [sallallahu `alayhi wa sallam]. And the possibility that the Messenger of God [sallallahu `alayhi wa sallam] asks forgiveness for them after his death is something impossible. This is due to the fact that after the death of a person all of his actions are discontinued - as the Messenger [sallallahu `alayhi wa sallam] has said - except for three: ongoing charity, knowledge which is being benefited from or a righteous child who prays for him. Hence there is no possibility for a man to ask forgiveness after his death for anyone, or even for himself, because his deeds have been discontinued." (End of quote.)

I say: this is a bold enterprise on the part of  Al-`Uthaymīn, we ask Allāh for safety. Its rebuttal is as follows:

The Objection of Al-`Uthaymīn and the Answer to it
Shaykh Maḥmūd Sa`īd Mamdūḥ