Talhah bin Ubaydullah bin Kariz (may Allah have mercy on him) narrated 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 alone; He has no partner)."
This is mursal; it has also been narrated from Malik through another chain as connected (mawsul), but its being connected is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9473