´Ibn Ubaid bin Umair narrated from his father:` "Ibn Umar was clinging on the two corners (in a manner that I had not seen any of the Companions of the Prophet doing) so I said: 'O Abu Abdur-Rahman! You are clinging on the two corners in a manner that I have not seen any of the Companions of the Prophet clining.' So he said: 'I do it because I heard the Messenger of Allah saying: "Touching them atones for sins." And I heard him saying: "Whoever performs Tawaf around this House seven times and he keeps track of it, then it is as if he freed a slave." And I heard him saying: "One foot is not put down, nor another raised except that Allah removes a sin from him and records a good merit for him."
Hadith Referenceسنن ترمذي / كتاب الحج عن رسول الله صلى الله عليه وسلم / 959
´It was narrated from Abdullah bin Ubaid bin Umair that a man said:` "O Abu abdur-Rahman, why do I only see you touching these two corners?" He said: "I heard the Messenger of Allah say: 'Touching them erases sins.' And I head him say: 'whoever circumambulates seven times, it is like freeing a slave.'"
Hafiz Muhammad Ameen
(1) "This is only in the Mujtaba." Imam Nasa'i rahimahullah wrote a lengthy book titled "As-Sunan al-Kubra." Due to its length, it was abridged and compiled as "Mujtaba Nasa'i." There is a difference of opinion regarding who compiled it: was it Imam Nasa'i himself or one of his students? There are some chapters that are found only in the Mujtaba and not in the Sunan al-Kubra. It is as if these were added to the Mujtaba. This chapter is also among those chapters.
(2) "Two corners" refers to the Hajar al-Aswad and the Rukn Yamani. The Hajar al-Aswad is the eastern corner and the Rukn Yamani is the southern corner. Since these two corners are on the original foundations, it is Sunnah to touch them.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2922