Talhah bin Ubaydullah bin Kariz (may Allah have mercy on him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The best supplication is the supplication of the Day of Arafah, and the best of what I and the prophets before me have said is 'La ilaha illallahu wahdahu la sharika lah' (There is no god but Allah, He is alone, He has no partner)."
This is mursal (the Successor narrates directly from the Prophet without naming the Companion).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8391