He 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh.
This refers to:
Ge 4:3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
8 And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.
9 And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper?
10 And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground.
Abel had the faith to do what God showed him to do, even though his older brother, Cain, was going about it the wrong way. His faith was so strong that even after Abel killed him, he, being now a dead man, was yet able to speak to God from out of the ground.