Maqaam Ibraaheem (the Station of Ibraaheem) is the stone on which he stood when the building became higher than he could reach. So his son put this famous stone there for him so that he could stand on it when the building grew tall… The footprints of al-Khaleel (Ibraaheem, peace be upon him) remained on the rock until the beginning of Islam. 

From al-Bidaayah wa’l-Nihaayah, 1/163 

Ibn Hajr said: What is meant by Maqaam Ibraaheem is the stone on which are the marks of his feet. 

Ibn Katheer said: 

The marks of his feet were clear on it and were well known; the Arabs were familiar with that during their jaahiliyyah and the Muslims also knew of that, as Anas ibn Maalik said: I saw the Maqaam on which were the marks of his toes and heels. 

But they disappeared because of people touching them with their hands. 

Ibn Jareer narrated that Qataadah said: “ ‘And take you (people) the Maqaam (place) of Ibraaheem (Abraham) [or the stone on which Ibraaheem (Abraham) stood while he was building the Ka‘bah] as a place of prayer (for some of your prayers, e.g. two Rak‘at after the Tawaaf of the Ka‘bah at Makkah)’ [al-Baqarah 2:125 – interpretation of the meaning]. This means that they were commanded to pray at that place; they were not commanded to touch it. This ummah went beyond what was prescribed for it, to an extent that no previous nation did. We have heard from those who saw the marks of his heels and toes on it, but this ummah kept touching them until they disappeared.” 

From Tafseer Ibn Katheer, 1/117 

Shaykh Ibn ‘Uthaymeen said: 

Undoubtedly Maqaam Ibraaheem is proven and that on which the glass enclosure is built is indeed Maqaam Ibraaheem. But the engraved marks that appear on it do not seem to be footprints, because what is well known from an historical point of view is that those footprints disappeared a long time ago. But these engraved marks were meant as a marker only, and we cannot be certain that these are the footprints of Ibraaheem (peace be upon him).