What is the ruling on climbing up Jabal al-Rahmah (the Mount of Mercy) on the day of ‘Arafah and praying atop this mountain?

It is not proven that the Prophet (blessings and peace of
Allah be upon him) encouraged climbing the mountain of ‘Arafaat which is
known to the people as Jabal al-Rahmah. It was not his way to climb this
mountain during Hajj or to take it as a ritual. He (blessings and peace of
Allah be upon him) said: “Learn from me your rituals (of Hajj).” The way
of the Rightly Guided Caliphs and all the Sahaabah was like that of the
Prophet (blessings and peace of Allah be upon him), and the same was true
of those who followed in their footsteps. They did not climb up to this
mountain during their Hajj and they did not take it as a ritual, following
the example of the Messenger of Allah (blessings and peace of Allah be
upon him). What is proven is that he (blessings and peace of Allah be upon
him) stood at the foot of this mountain at the large rocks and he said: “I
have stood here and all of ‘Arafah is the place of standing, and avoid the
valley of ‘Aranah. Hence many of the scholars, such as Imam al-Nawawi,
Shaykh al-Islam Ibn Taymiyah and Shaykh Siddeeq Khan, said: Climbing this
mountain during Hajj as if it were a ritual is an innovation. The Prophet
(blessings and peace of Allah be upon him) said: “Whoever does an action
that is not part of this matter of ours will have it rejected.” It was not
his practice to offer any naafil prayer in the standing at ‘Arafah; rather
he limited it to praying Zuhr and ‘Asr in Masjid Namirah, joining and
shortening the prayer. He did not take any place for prayer in what is
called Jabal al-Rahmah in which those who climb this mountain should offer
naafil or obligatory prayers on the day of ‘Arafah. Rather, after praying
Zuhr and ‘Asr, he busied himself with remembering Allah (dhikr), reciting
tasbeeh (“Subhaan Allah”), tahleel (Laa ilaaha ill-Allaah), tahmeed
(al-hamdu Lillah), takbeer (Allahu akbar) and Talbiyah, calling upon his
Lord and beseeching Him, until the sun set. So taking a place on this
mountain in which to pray by those who climb up it is an innovation
introduced by the ignorant. 

And Allah is the source of strength; may Allah send blessings
and peace upon our Prophet Muhammad and his family and Companions. End
quote. 

Standing Committee for Academic Research and Issuing Fatwas. 

Shaykh Ibraaheem ibn Muhammad Aal al-Shaykh, Shaykh ‘Abd
al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn
Munayyi’ 

Fataawa al-Lajnah al-Daa’imah,
11/206-208 

Shaykh Ibn Uthaymeen (may Allah have mercy on him) said: 

Climbing up the mountain of ‘Arafaat is not something that is
prescribed in Islam; rather if the people take it as an act of worship it is
an innovation; it is not permissible for people to believe that this is an
act of worship or to do it on the basis that it is an act of worship. The
Messenger (blessings and peace of Allah be upon him) was the keenest of all
people to do good and was the most effective of all people in conveying the
message and he was the most knowledgeable of all people of the religion of
Allah. He did not climb this mountain or instruct anyone to do so, and he
did not approve of anyone climbing it as far as I know. Based on that,
climbing this mountain is not prescribed; rather the Messenger of Allah
(blessings and peace of Allah be upon him) said, when he stood behind it on
the eastern side: “I have stood here and all of ‘Arafah is the place of
standing.” It is as if he (blessings and peace of Allah be upon him) was
indicating that each person should stand in his own spot and not push and
shove in order to reach the place where the Messenger (blessings and peace
of Allah be upon him) stood. End quote. 

Majmoo’ Fataawa Ibn ‘Uthaymeen (23/32)