The Iron - 57:19 Those who believe in Allah and His messenger are the righteous and the martyrs. For them is a reward and light with their Lord. And those who disbelieve and deny Our verses are the inhabitants of Hell.
The martyrs/witnesses are those who bear witness to the Truth/Qur'an, that Allah is the only God:
The Family Of Imran/3:64 "Say, "People of the Book! Come to a principle which is the same between us and you: "Let us serve none but Allah and associate with Him nothing in common, and let not one of us take the other as gods after Allah."If they refuse, say, "Bear witness, we are Muslims!"

The Letter Qaf/50:37 In this is a message for anyone who has intelligence or who listens as a witness.

The Women/4:166 But Allah bears witness to what He has sent down to you. He sent it down with His knowledge. The angels also bear witness to it. Allah is sufficient as a witness.

The Family Of Imran/3:18 Allah testifies that there is no god but Him, and the angels and the learned who observe justice. There is no god but Him. He is the Exalted, the Wise.

The Table Spread/5:83 "When they hear what has come down to the Messenger, you see their eyes overflowing with tears because they recognize the truth. They say, "Our Lord, we believe; count us among the witnesses."
The Table Spread/5:82 "Among mankind you will find the Jews and the polytheists the worst enemies of the believers. And closest in love to the believers are those who say: "We are Christians". For among them are priests and monks who are not arrogant."
The Table Spread/5:44 We sent down the Torah in which is guidance and light, and the Muslim prophets used to judge between the Jews by it. And the rabbis and the scribes used to judge by what they were commanded from the Book of Allah, and they were witnesses over it. Do not honor the people, honor Me, and do not sell My verses at a cheap price. Those who do not judge by what Allah has revealed are disbelievers.
The Table Spread/5:113 We ask that we may eat of it, and that our hearts may be calmed, and that we may know that you tell us the truth, and that we may bear witness to it.

The Cattle/6:19 "Ask: "Whose testimony is the greatest?"Say: "Allah is the witness between me and you. I have been given this Qur´an to warn you and all those to whom it reaches. Do you bear witness that there is another god besides Allah?"Say: "I do not bear witness to that,"and then say: "He is the only God, and I am far from that which you associate partners with."

The Heights/7:172 "When your Lord brings forth the offspring from the loins of the sons of Adam, He makes them witnesses to themselves: "Am I not your Lord?"They say, "Yes, we bear witness."So on the Day of Resurrection you will not be able to say: "We were unaware of this"."

The Prophets/21:56 "He said, "Indeed your Lord is the Lord of the heavens and the earth; He created them separately. I am of those who bear witness to this."