Musannaf Ibn Abi Shayba 17:1363

Ibn ‘Ulayyah narrated to us, from Ayyub, from Humayd ibn Hilal, that Mu’adh spat one day to his right. Then he said, "Alas, I have not done this since I accompanied the Prophet, may Allah bless him and grant him peace," or he said, "since I embraced Islam."