Musnad Ahmad 8:71

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Ali ibn Zayd, who said: I heard Sa‘id ibn al-Musayyib say: I said to Sa‘d ibn Malik, "You are a man who has sharpness, and I want to ask you." He said, "What is it?" He said: I said, "The hadith of ‘Ali." He said: So he said, "The Prophet, may Allah bless him and grant him peace, said to ‘Ali: 'Are you not pleased to be to me in the position of Aaron to Moses?'" He said, "I am pleased, I am pleased." Then he said, "Yes, yes."