the Messenger of Allah (saas) said: "The Greatest Name of Allah is in these two Verses: And your Ilah (God) is One Ilah (God - Allah), La Ilaha Illa Huwa (none has the right to be worshipped but He), the Most Gracious, the Most Merciful.' And at the beginning of Surah Al 'Imran."  

قَالَ رَسُولُ اللَّهِ ﷺ اسْمُ اللَّهِ الأَعْظَمُ فِي هَاتَيْنِ الآيَتَيْنِ {وَإِلَهُكُمْ إِلَهٌ وَاحِدٌ لاَ إِلَهَ إِلاَّ هُوَ الرَّحْمَنُ الرَّحِيمُ} وَفَاتِحَةِ سُورَةِ آلِ عِمْرَانَ  

The Greatest Name of Allah, if He is called by which He will respond, is in three Surah: Al-Baqarah, Al 'Imran and Ta-Ha. (Hasan)Another chain for something similar from Al-Qasim, from Abu Umamah, from the Prophet (saas).  

اسْمُ اللَّهِ الأَعْظَمُ الَّذِي إِذَا دُعِيَ بِهِ أَجَابَ فِي سُوَرٍ ثَلاَثٍ الْبَقَرَةِ وَآلِ عِمْرَانَ وَطَهَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ إِبْرَاهِيمَ الدِّمَشْقِيُّ حَدَّثَنَا عَمْرُو بْنُ أَبِي سَلَمَةَ قَالَ ذَكَرْتُ ذَلِكَ لِعِيسَى بْنِ مُوسَى فَحَدَّثَنِي أَنَّهُ سَمِعَ غَيْلاَنَ بْنَ أَنَسٍ يُحَدِّثُ عَنِ الْقَاسِمِ عَنْ أَبِي أُمَامَةَ عَنِ النَّبِيِّ ﷺ نَحْوَهُ  

"The Prophet (saas) heard a man say: 'Allahumma! Inni as'aluka bi-annaka Antallahul-Ahadus-Samad, alladhi lam yalid wa lam yuwlud, wa lam yakun lahu kufuwan ahad (O Allah! I ask You by virtue of Your being Allah, the One, the Self-Sufficient Master, Who begets not nor was begotten, and there is none co-equal or comparable to Him).' The Messenger of Allah (saas) said: 'He has asked Allah by His Greatest Name, which if He is asked thereby He gives, and if He is called upon thereby He answers.'"  

سَمِعَ النَّبِيُّ ﷺ رَجُلاً يَقُولُ اللَّهُمَّ إِنِّي أَسْأَلُكَ بِأَنَّكَ أَنْتَ اللَّهُ الأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ فَقَالَ رَسُولُ اللَّهِ ﷺ لَقَدْ سَأَلَ اللَّهَ بِاسْمِهِ الأَعْظَمِ الَّذِي إِذَا سُئِلَ بِهِ أَعْطَى وَإِذَا دُعِيَ بِهِ أَجَابَ  

"The Prophet (saas) heard a man say: 'Allahumma! Inni as'aluka bi-anna lakal-hamd. La ilaha illa Anta, wahdaka la sharika laka. Al-Mannan. Badi'us-samawati wal-ard. Dhul-jalali wal-ikram (O Allah! I ask You by virtue of the fact that all praise is due to You; none has the right to be worshiped but You alone, and You have no partner or associate, the Bestower, the Originator of the heavens and the earth, the Possessor of majesty and honor.' He (saas) said: 'He has asked Allah by His Greatest Name which, if He is asked thereby He gives and if He is called upon thereby He answers.'"  

سَمِعَ النَّبِيُّ ﷺ رَجُلاً يَقُولُ اللَّهُمَّ إِنِّي أَسْأَلُكَ بِأَنَّ لَكَ الْحَمْدَ لاَ إِلَهَ إِلاَّ أَنْتَ وَحْدَكَ لاَ شَرِيكَ لَكَ الْمَنَّانُ بَدِيعُ السَّمَوَاتِ وَالأَرْضِ ذُو الْجَلاَلِ وَالإِكْرَامِ فَقَالَ لَقَدْ سَأَلَ اللَّهَ بِاسْمِهِ الأَعْظَمِ الَّذِي إِذَا سُئِلَ بِهِ أَعْطَى وَإِذَا دُعِيَ بِهِ أَجَابَ  

"I heard the Messenger of Allah (saas) say: 'Allahumma! Inni asa'luka bismikat-tahirit-tayyibil-mubarak al-ahabbi ilaika, alladhi idha du'ita bihi ajabta, wa idha su'ilta bihi a'taita, wa idhasturhimta bihi rahimta, wa idhastufrijta bihi farrajta (O Allah! I ask You by Your pure, good and blessed Name which is most beloved by You, which if You are called thereby You answer, and if You as asked thereby You give, if You are asked for mercy thereby You bestow mercy, and if You are asked for relief (from distress) thereby You grant relief.'"She said: "He said one day: 'O 'Aishah, do you know that Allah has told me the Name which, if He is called thereby, He responds?' I said: 'O Messenger of Allah, may my father and mother be ransomed for you! Teach it to me.' He said: 'You should not learn it, O 'Aishah.' So I moved aside and sat for a while, then I got up and kissed his head, then I said: 'O Messenger of Allah, teach it to me.' He said: 'You should not learn it, O 'Aishah, and I should not teach it to you, for you should not ask for any worldly things thereby.'" She said: "So I got up and performed ablution, then I prayed two Rak'ah, then I said: 'O Allah, I call upon Allah, and I call upon You, Ar-Rahman (the Most Gracious), and I call upon You, Al-Barr Ar-Rahim (The Most Kind, the Most Merciful), and I call upon You by all Your beautiful Names, those that I know and those that I do not know, (asking) that You forgive me and have mercy on me.' The Messenger of Allah (saas) smiled, then he said: 'It is among the names by which you called upon (Allah).'"  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ اللَّهُمَّ إِنِّي أَسْأَلُكَ بِاسْمِكَ الطَّاهِرِ الطَّيِّبِ الْمُبَارَكِ الأَحَبِّ إِلَيْكَ الَّذِي إِذَا دُعِيتَ بِهِ أَجَبْتَ وَإِذَا سُئِلْتَ بِهِ أَعْطَيْتَ وَإِذَا اسْتُرْحِمْتَ بِهِ رَحِمْتَ وَإِذَا اسْتُفْرِجْتَ بِهِ فَرَّجْتَ قَالَتْ وَقَالَ ذَاتَ يَوْمٍ يَا عَائِشَةُ هَلْ عَلِمْتِ أَنَّ اللَّهَ قَدْ دَلَّنِي عَلَى الاِسْمِ الَّذِي إِذَا دُعِيَ بِهِ أَجَابَ قَالَتْ فَقُلْتُ يَا رَسُولَ اللَّهِ بِأَبِي أَنْتَ وَأُمِّي فَعَلِّمْنِيهِ قَالَ إِنَّهُ لاَ يَنْبَغِي لَكِ يَا عَائِشَةُ قَالَتْ فَتَنَحَّيْتُ وَجَلَسْتُ سَاعَةً ثُمَّ قُمْتُ فَقَبَّلْتُ رَأْسَهُ ثُمَّ قُلْتُ يَا رَسُولَ اللَّهِ عَلِّمْنِيهِ قَالَ إِنَّهُ لاَ يَنْبَغِي لَكِ يَا عَائِشَةُ أَنْ أُعَلِّمَكِ إِنَّهُ لاَ يَنْبَغِي لَكِ أَنْ تَسْأَلِي بِهِ شَيْئًا مِنَ الدُّنْيَا قَالَتْ فَقُمْتُ فَتَوَضَّأْتُ ثُمَّ صَلَّيْتُ رَكْعَتَيْنِ ثُمَّ قُلْتُ اللَّهُمَّ إِنِّي أَدْعُوكَ اللَّهَ وَأَدْعُوكَ الرَّحْمَنَ وَأَدْعُوكَ الْبَرَّ الرَّحِيمَ وَأَدْعُوكَ بِأَسْمَائِكَ الْحُسْنَى كُلِّهَا مَا عَلِمْتُ مِنْهَا وَمَا لَمْ أَعْلَمْ أَنْ تَغْفِرَ لِي وَتَرْحَمَنِي قَالَتْ فَاسْتَضْحَكَ رَسُولُ اللَّهِ ﷺ ثُمَّ قَالَ إِنَّهُ لَفِي الأَسْمَاءِ الَّتِي دَعَوْتِ بِهَا