Sermon 20 of Nahj al-Balagha: Death and taking lessons from it

Indeed, if you could see that which has been seen by those of you who have died, you would be puzzled and troubled. Then you would have listened and obeyed; but what they have seen is yet curtained off from you. Shortly, the curtain would be thrown off. You have been shown, provided you see and you have been made to listen provided you listen, and you have been guided if you accept guidance. I spoke unto you with truth.

You have been called aloud by (instructive) examples and warned through items full of warnings. After the heavenly messengers (angels), only man can convey message from Allah. (So what I am conveying is from Allah).
 

Alternative Sources for Sermon 20
(1) Al-Kulayni, Usul al-Kafi, I, 405