The Prohibition of asking for Forgiveness for Hypocrites
Allah says to His Prophet that hypocrites are not worthy of seeking forgiveness for them and that if he asks Allah to forgive them seventy times, Allah will not forgive them. The number seventy here was mentioned to close the door on this subject, for Arabs use this number when they exaggerate, not that they actually mean seventy or more than seventy. Ash-Sha`bi said that when `Abdullah bin Ubayy was dying, his son went to the Prophet and said to him, "My father has died, I wish you could attend him and pray the funeral prayer for him." The Prophet said,
«مَا اسْمُك»
("What is you name) He said, "Al-Hubab bin `Abdullah." The Prophet said,
«بَلْ أَنْتَ عَبْدُاللهِ بْنُ عَبْدِاللهِ إِنَّ الْحُبَابَ اسْمَ شَيْطَان»
(Rather, you are `Abdullah bin `Abdullah, for Al-Hubab is a devil's name.) The Prophet went along with him, attended his father's funeral, gave him his shirt as a shroud and prayed the funeral prayer for him. He was asked, "Would you pray on him, when he is a hypocrite" He said,
«إِنَّ اللهَ قَالَ:
إِن تَسْتَغْفِرْ لَهُمْ سَبْعِينَ مَرَّةً
وَلَأسْتَغْفِرَنَّ لَهُمْ سَبْعِينَ وَسَبْعِينَ وَسَبْعِين»
(Allah said,(...(and even) if you ask seventy times for their forgiveness...) Verily, I will ask Allah to forgive them seventy times and seventy more and seventy more.)" Similar narrations were collected from `Urwah bin Az-Zubayr, Mujahid, Qatadah bin Di`amah and Ibn Jarir.
Tafsir Ibn Kathir (abridged, English).