Musnad Ahmad 937:233

Zayd ibn al-Hubab narrated to us, Abd al-Rahman ibn Thawban narrated to us, from his father, from Makhul, from Umar ibn Nu'aym, from Usamah ibn Salman, from Abu Dharr, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Verily, Allah forgives His servant as long as the veil does not fall." They said: "O Messenger of Allah, and what is the veil?" He said: "That the soul dies while it is associating partners with Allah (shirk)."