Who was it that defined the boundaries of the Haram?

The one who first defined the boundaries of the Haram was the Prophet of Allah Ibraaheem (peace and blessings of Allah be upon him and upon our Prophet), by the command of Allah and on the basis of what He revealed to him.

An-Nawawi (may Allah have mercy on him)said:

Al-Azraqi and others stated, with their isnaads, that Ibraaheem al-Khaleel (peace be upon him) defined the boundary and set up markers for it, and Jibreel (peace be upon him) showed him where to place them. Then our Prophet (blessings and peace of Allah be upon him) was instructed to renew them, and they were subsequently renewed by ‘Umar, then ‘Uthmaan, then Mu‘aawiyah (may Allah be pleased with them all), and until the present they have remained well-known, visible and clear, praise be to Allah. At the end of Kitaab Makkah, al-Azraqi said: With regard to the boundary markers of the Haram that are at the top of ath-Thaniyyah, whatever is in front of them is within the Haram zone, and whatever is behind them is outside the Haram zone. Part of al-A‘shaash is outside the Haram zone and part of it is inside the Haram zone.

End quote from Sharh al-Muhadhdhab (7/463)

It was narrated from Ibn ‘Abbaas (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) sent Tameem ibn Asad al-Khuzaa‘i to renew the boundary markers of the Haram. They had been placed by Ibraaheem (peace be upon him), and Jibreel (peace be upon him) showed him where to put them. Narrated by Abu Na‘eem in Ma‘rifat as-Sahaabah (1/425). Its isnaad was classed as hasan by al-Haafiz Ibn Hajar (may Allah have mercy on him) in al-Isaabah fi Tamyeez as-Sahaabah, in his biography of Tameem ibn Asad al-Khuzaa‘i (may Allah be pleased with him) (1/367)

And Allah knows best.