The Injil was the revelation sent down to prophet Isa (Jesus). The Injil is one of the four revelations recognised by Islam. The 4 revelations being:
The word Injil is often used in the Qur'an, the hadith and early Muslim documents to refer to the revelations made by God to Jesus - it had never been compiled in the form of a literal book, revealed only as an oral book:
نَزَّلَ عَلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِّمَا بَيْنَ يَدَيْهِ وَأَنزَلَ التَّوْرَاةَ وَالْإِنجِيلَ
He has sent down upon you, [O Muhammad], the Book in truth, confirming what was before it. And He revealed the Torah and the Gospel. (Qur'an 3:3)
From what the Qur'an tells us, this message was revealed to Isa (as) who went through great hardship to spread the message of Allah. Allah supported him with miracles to evidence his prophethood to his people:
Speaking from the cradle
Creating birds from clay
Healing the blind leaper
Power over death
Table of food from heaven
And so on.
Below is a verse from the Qur'an talking about the table of food from the heavens.
قَالُوا نُرِيدُ أَن نَّأْكُلَ مِنْهَا وَتَطْمَئِنَّ قُلُوبُنَا
وَنَعْلَمَ أَن قَدْ صَدَقْتَنَا وَنَكُونَ عَلَيْهَا مِنَ الشَّاهِدِينَ
They said, "We wish to eat from it and let our hearts be reassured and
know that you have been truthful to us and be among its witnesses."
As you can see, his followers asked him to do a miracle for them to believe the words he speaks.
The Injil was revealed to Isa (as) comparable to how the Qur'an was revealed to the Prophet Muhammad (saw):
فَآتَيْنَا الَّذِينَ آمَنُوا مِنْهُمْ أَجْرَهُمْ ۖ وَكَثِيرٌ
Then We sent following their footsteps Our messengers and followed
[them] with Jesus, the son of Mary, and gave him the Gospel. And We
placed in the hearts of those who followed him compassion and mercy
and monasticism, which they innovated; We did not prescribe it for
them except [that they did so] seeking the approval of Allah . But
they did not observe it with due observance. So We gave the ones who
believed among them their reward, but many of them are defiantly
disobedient. (Qur'an 57:27)
The revelation to Isa (as) known as the Injil has not been transmitted faithfully to us - undergoing changes, necessitating yet another messenger, the Prophet Muhammad (saw), to convey the truth.