Indeed, there is no clear evidence from a verse in the Qur’an or an authentic hadith who the sacrifice was, neither is there a consensus.
In Surah As-Saffat, Allah gives tidings of Isma’el, then there is a narration of the story of sacrifice, then Allah gives tidings of Is’haq, followed by Ya’qoob:
وَقَالَ إِنِّي ذَاهِبٌ إِلَىٰ رَبِّي سَيَهْدِينِ رَبِّ هَبْ لِي مِنَ الصَّالِحِينَ فَبَشَّرْنَاهُ بِغُلَامٍ حَلِيمٍ فَلَمَّا بَلَغَ مَعَهُ السَّعْيَ قَالَ يَا بُنَيَّ إِنِّي أَرَىٰ فِي الْمَنَامِ أَنِّي أَذْبَحُكَ فَانْظُرْ مَاذَا تَرَىٰ ۚ قَالَ يَا أَبَتِ افْعَلْ مَا تُؤْمَرُ ۖ سَتَجِدُنِي إِنْ شَاءَ اللَّهُ مِنَ الصَّابِرِينَ فَلَمَّا أَسْلَمَا وَتَلَّهُ لِلْجَبِينِ وَنَادَيْنَاهُ أَنْ يَا إِبْرَاهِيمُ قَدْ صَدَّقْتَ الرُّؤْيَا ۚ إِنَّا كَذَٰلِكَ نَجْزِي الْمُحْسِنِينَ إِنَّ هَٰذَا لَهُوَ الْبَلَاءُ الْمُبِينُ وَفَدَيْنَاهُ بِذِبْحٍ عَظِيمٍ وَتَرَكْنَا عَلَيْهِ فِي الْآخِرِينَ سَلَامٌ عَلَىٰ إِبْرَاهِيمَ كَذَٰلِكَ نَجْزِي الْمُحْسِنِينَ إِنَّهُ مِنْ عِبَادِنَا الْمُؤْمِنِينَ وَبَشَّرْنَاهُ بِإِسْحَاقَ نَبِيًّا مِنَ الصَّالِحِينَ وَبَارَكْنَا عَلَيْهِ وَعَلَىٰ إِسْحَاقَ ۚ وَمِنْ ذُرِّيَّتِهِمَا مُحْسِنٌ وَظَالِمٌ لِنَفْسِهِ مُبِينٌ
And [then] he said, "Indeed, I will go to [where I am ordered by] my Lord; He will guide me. My Lord, grant me [a child] from among the righteous." So We gave him good tidings of a forbearing boy. And when he reached with him [the age of] exertion, he said, "O my son, indeed I have seen in a dream that I [must] sacrifice you, so see what you think." He said, "O my father, do as you are commanded. You will find me, if Allah wills, of the steadfast." And when they had both submitted and he put him down upon his forehead, We called to him, "O Abraham, You have fulfilled the vision." Indeed, We thus reward the doers of good. Indeed, this was the clear trial. And We ransomed him with a great sacrifice, And We left for him [favorable mention] among later generations: "Peace upon Abraham." Indeed, We thus reward the doers of good. Indeed, he was of Our believing servants. And We gave him good tidings of Isaac, a prophet from among the righteous. And We blessed him and Isaac. But among their descendants is the doer of good and the clearly unjust to himself.
Qur’an 37:99-113 (Sahih International translation)
It may be concluded that the above-mentioned verses are relating the story in sequence. If so, then the sacrifice was Isma'el. However, it is well known that the word and in Arabic does not necessarily indicate sequence. For example, in Qur’an 54:18, Allah says: “'Aad denied; and how [severe] were My punishment and warning.” Allah’s punishment occurred after His warning, but the verse mentions the word "punishment" first (not in sequence).
When the Qur'an mentions the good tidings of Is'haq:
وَلَقَدْ جَاءَتْ رُسُلُنَا إِبْرَاهِيمَ بِالْبُشْرَىٰ قَالُوا سَلَامًا ۖ قَالَ سَلَامٌ ۖ فَمَا لَبِثَ أَنْ جَاءَ بِعِجْلٍ حَنِيذٍ فَلَمَّا رَأَىٰ أَيْدِيَهُمْ لَا تَصِلُ إِلَيْهِ نَكِرَهُمْ وَأَوْجَسَ مِنْهُمْ خِيفَةً ۚ قَالُوا لَا تَخَفْ إِنَّا أُرْسِلْنَا إِلَىٰ
قَوْمِ لُوطٍ وَامْرَأَتُهُ قَائِمَةٌ فَضَحِكَتْ فَبَشَّرْنَاهَا بِإِسْحَاقَ وَمِنْ وَرَاءِ إِسْحَاقَ يَعْقُوبَ قَالَتْ يَا وَيْلَتَىٰ أَأَلِدُ وَأَنَا عَجُوزٌ وَهَٰذَا بَعْلِي شَيْخًا ۖ إِنَّ هَٰذَا لَشَيْءٌ عَجِيبٌ
And certainly did Our messengers come to Abraham with good tidings; they said, "Peace." He said, "Peace," and did not delay in bringing [them] a roasted calf. But when he saw their hands not reaching for it, he distrusted them and felt from them apprehension. They said, "Fear not. We have been sent to the people of Lot." And his Wife was standing, and she smiled. Then We gave her good tidings of Isaac and after Isaac, Jacob. She said, "Woe to me! Shall I give birth while I am an old woman and this, my husband, is an old man? Indeed, this is an amazing thing!"
Qur’an Hud 11:69-72 (Sahih International translation)
It is mentioned that Ibrahim was informed about Is'haq, and after Is'haq comes Ya'qoub. We know now that Ya'qoub was Is'haq's son, but from the verse it could be that he would be Is'haq's son, or another son of Ibrahim after Is'haq, and so on. There is no explicit mention in the verse that Ya'qoub would be Is'haq's son.
The most famous hadith that mentions Isma’el by name was documented in Musnad Ahmad:
إن جبريل ذهب بإبراهيم إلى جمرة العقبة فعرض له الشيطان فرماه بسبع حصيات فساخ ثم أتى الجمرة الوسطى فعرض له الشيطان فرماه بسبع حصيات فساخ ثم أتى الجمرة القصوى فعرض له الشيطان فرماه بسبع حصيات فساخ فلما أراد إبراهيم أن يذبح ابنه اسماعيل قال لأبيه: يا أبت أوثقني لا أضطرب فينتضح عليك من دمي إذا ذبحتني فشده فلما أخذ الشفرة فأراد أن يذبحه نودي من خلفه أن يا إبراهيم قد صدقت الرؤيا
Jibril went with Ibrahim to the Jamrah of Aqaba, the Shaitan appeared to him, so Ibrahim threw seven pebbles at the Shaitan. The Shaitan disappeared. When Ibrahim reached the middle jamrah, the Shaitan re-appeared to him, so Ibrahim threw seven pebbles at the Shaitan. The Shaitan disappeared again. When Ibrahim reached the farthest jamrah, , the Shaitan re-appeared to him, so Ibrahim threw seven pebbles at the Shaitan. The Shaitan disappeared again. When Ibrahim wanted to slay his son Isma’el, Isma’el told his father: O father, tie me up so that if I jitter, my blood does not get on you when you slay me. So Ibrahim tightened the knot, then when he took the knife to slay him, he heard from behind: "O Abraham, You have fulfilled the vision."
— Musnad Ahmad, Hadith #144283 (my own translation, so treat with care).
This hadith is narrated through the authority of ‘Ataa’ ibn Al-Sa’eb through ‘Abdullah ibn ‘Abbas. Although ‘Ataa’ ibn Al-Sa’eb is considered a trusted source, during his last years, his memory became weaker, and as such with this particular hadith which is not definite when he narrated it, it is not considered an authentic hadith by some scholars.
There is a number of other hadiths that say the sacrifice was Is’haq, but all of them do not go above the level weak (weak, very weak, or fabricated). Thus, in terms of hadiths, there is no conclusive evidence that the sacrifice was either Isma’el or Is’haq.
There are several books that discussed the topics from Zahrat At-Tafasir to Al-Dorr Al-Manthour to At-Tafsir Al-Kabir to Zaad Al-Mi'aad to Al-Bedaya Wa Al-Nehaya, among others. There is a difference in opinions. In summary, Abdullah ibn 'Abbas, 'Abdullah ibn Mas'ood, 'Umar ibn Al-Khattab, and 'Ali ibn Abi Talib, Al-Zohry, among others, said that the sacrifice was Is'haq. Abdullah ibn 'Umar, Sa'id ibn Al-Musayyib, Hassan Al-Basry, Mujahid, 'Ataa' ibn Abi Rabah, among others, said that the sacrifice was Isma'el.
We know from the Seerah that Isma'el was in Mecca, and that Is'haq was in Jerusalem, and that the sacrifice act took place in Mecca. However, from a scholarly point of view, this is not enough evidence to draw a conclusion.
The bottom line is we do not know which son was the son of sacrifice, neither should we be too bothered about it. As Muslims, we believe both Isma'el and Is'haq were prophets of Allah, and that Ibrahim was a mighty prophet of Allah. We also believe that Allah tests his prophets at the highest level of tests (as their faith is also at the highest level). It was a test from Allah to Ibrahim and his son, and they passed. May Allah's blessings be on all of them.