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
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: I saw the Messenger of Allah (peace and blessings be upon him) at Arafah supplicating with his hands raised towards his chest, like a poor person asking for food.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9474
Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Most of my supplication, and of the prophets before me, at Arafah is: 'There is no god but Allah alone, He has no partner; His is the dominion and His is the praise, and He has full power over all things. O Allah, place light in my heart, my hearing and my sight. O Allah, open my chest for me and make my affair easy for me. I seek refuge from the whisperings of the chest, from the scattering of affairs and from the trial of the grave. I seek refuge from the evil of what enters into the day and the night, from the calamities of time, and from the evil of what the winds blow with.'"
It is narrated from Abu Shu'bah: I kept my eyes fixed on Sayyiduna Ibn Umar (may Allah be pleased with them both) while he was at Arafah, listening to what he was reciting; he said that he did not recite anything more than 'La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamd, wa huwa 'ala kulli shay'in qadir' (There is no god but Allah alone, He has no partner; His is the dominion and His is the praise, and He has full power over all things).
Only Musa bin Ubaydah narrated it, and he is da'if (weak); and his brother did not meet Ali (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9475