Musnad Ahmad 5:791

Abu Sa'id narrated to us, Sa'id ibn Salamah ibn Abi al-Husam narrated to us, 'Abdullah ibn Muhammad ibn 'Aqil narrated to us, from Muhammad ibn 'Ali al-Akbar, that he heard his father, 'Ali ibn Abi Talib, may Allah be pleased with him, say: The Messenger of Allah, may Allah bless him and grant him peace, said: "I have been given four things that no one among the prophets of Allah was given before me: I have been given the keys of the earth, I have been named Ahmad, the earth has been made a means of purification for me, and my nation has been made the best of nations."