What is the authenticity of the Hadith: "The best remembrance (dhikr) is 'La ilaha illa Allah' (There is no god but Allah), and the best of what I and the Prophets before me have said is 'La ilaha illa Allah,' and it is the word of Tawhid (Oneness of God) and sincerity, and it is Allah's Greatest Name (Ism al-A'dham)"? And what is the reason why scholars do not circulate it when discussing the Greatest Name?

The prophetic hadiths stated that the best remembrance (dhikr) is "La ilaha illa Allah (There is no god but Allah)," and the best supplication (du'a) is "Alhamdulillah (Praise be to Allah)." Furthermore, "La ilaha illa Allahu wahdahu la sharika lahu, lahul mulku wa lahul hamdu wa huwa 'ala kulli shay'in qadir (There is no god but Allah, alone, without partner. To Him belongs sovereignty, and to Him belongs praise, and He is over all things competent)" is the best of what the prophets said. As for describing the word of Tawhid (monotheism) as "Allah's Greatest Name," this is not a raised hadith (i.e., directly from the Prophet), but rather a statement that was "said" and transmitted by scholars.