Who told Cain and Abel of Sacrificial rituals or offerings? In Genesis 4 Cain and Abel gave offerings (or sacrifice). Who instructed them to give this offering or sacrifice? Was it a burnt offering?
The Church Fathers tell us that the idea of offering sacrifice to God was placed in the conscience of man from the beginning:


  In the course of time Cain brought an offering of the fruits of the earth to the Lord
  
  Consider how the Lord of nature added knowledge to conscience.  After
  all, who brought this to our understanding?  It was none other than
  knowledge associated with our conscience.  The text says, He brought
  an offering of the fruits of the earth to the Lord [Genesis 4:3].  He knew and
  understood that he should offer from his own possessions some produce
  to God as to his master, not because God needs them, but for the
  purpose of demonstrating his gratitude as being himself a beneficiary
  of such kindness.  God, you see, is proof against need, and depends on
  nothing we have to offer; but in his ineffable love he shows
  considerateness for us, and for the sake of our salvation he allows
  these things to happen so that the knowledge of the Lord may be for
  the human race a school of virtue.
  
  John Chrysostom (4th c.), Homilies on Genesis, Homily XVIII