"I was sitting with the Messenger of Allah ﷺ and a man was standing and praying. When he bowed, prostrated and recited the tashahhud, he supplicated, and in his supplication he said: "Allahumma inni as'aluka bi-anna lakal-hamd, lailaha illa ant, al-mannanu badi'us-samawati wal-ard, ya dhal-jalali wal-ikram! Ya hayyu ya qayyum! Inni as'aluka. (O Allah, indeed I ask You since all praise is due to You, there is none worthy of worship but You, the Bestower, the Creator of the heavens and earth, O Possessor of majesty and honor, O Ever-living, O-Eternal, I ask of You.)' The Prophet ﷺ said: 'Do you know what he has supplicated with?' They said: "Allah (SWT) and His Messenger know best." He said: 'By the One in Whose Hand is my soul, he called upon Allah by His greatest Name, which, if He is called by it, He responds, and if He is asked by it, He gives.'" (Using translation from Nasāʾī 1300)  

كُنْتُ مَعَ رَسُولِ اللهِ ﷺ جَالِسًا يَعْنِي وَرَجُلٌ قَائِمٌ يُصَلِّي فَلَمَّا رَكَعَ وَسَجَدَ وَتَشَهَّدَ دَعَا فَقَالَ فِي دُعَائِهِ اللهُمَّ إِنِّي أَسْأَلُكَ بِأَنَّ لَكَ الْحَمْدُ لَا إِلَهَ إِلَّا أَنْتَ الْمَنَّانُ بَدِيعُ السَّمَوَاتِ وَالْأَرْضِ يَا ذَا الْجِلَالِ وَالْإِكْرَامِ يَا حَيُّ يَا قَيُّومُ إِنِّي أَسْأَلُكُ فَقَالَ النَّبِيُّ ﷺ لِأَصْحَابِهِ «تَدْرُونَ بِمَا دَعَا» قَالُوا اللهُ وَرَسُولُهُ أَعْلَمُ قَالَ «وَالَّذِي نَفْسِي بِيَدِهِ لَقَدْ دَعَا بِاسْمِهِ الْعَظِيمِ الَّذِي إِذَا دُعِيَ بِهِ أَجَابَ وَإِذَا سُئِلَ بِهِ أَعْطَى»  

Mihjan bin Al-Adra' narrated to him that the Messenger of Allah ﷺ entered the masjid and there was a man who had finished his prayer and he was reciting the tashahhud. He said: "Allahumma inni as'aluka ya Allah! Bi-annakal-Wahidul-Ahad us-Samad, alladhi lam yalid wa lam yowled, wa lam yakun lahu kufuwan ahad, an taghfirali dhunubi, innaka antal-Ghafurur-Rahim (O Allah, I ask of You, O Allah, as You are the One, the Only, the Self-Sufficient Master, Who begets not nor was He begotten, and there is None equal or comparable to Him, forgive me my sins, for You are the Oft-Forgiving, Most Merciful.)" The Messenger of Allah ﷺ said: "He has been forgiven," three times. (Using translation from Nasāʾī 1301)  

أَنَّ رَسُولَ اللهِ ﷺ دَخَلَ الْمَسْجِدَ إِذَا رَجُلٌ قَدْ صَلَّى وَهُوَ يَتَشَهَّدُ فَقَالَ اللهُمَّ إِنِّي أَسْأَلُكُ يَا اللهُ الْوَاحِدُ الْأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوبِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمِ فَقَالَ رَسُولُ اللهِ ﷺ «قَدْ غُفِرَ لَهُ» ثَلَاثًا