Kabaah is the Holiest place for Muslims. Holy Kabaah is in square shape so it has four corners. Each of them have its own significance but two corners, Black Stone and Yamani Corner have more importance than others. Rukn al-Yamani is the southern west corner of Holy Kabaah. It is parallel to Black Stone corner. Yamani Corner is in the direction of Yemen country that is the reason of calling it Rukn Al-Yamani.
This corner was built on the first foundation of the Holy Kabaah Raised by Hazrat Ibrahim (A.S) and Hazrat Ismaeel (A.S). During the time when Masjid al-Aqsa is the Qiblah for Muslims, Holy Prophet ﷺ used to offer His prayers in front of Rukn al-Yamani so that he can face both Kabaah and Masjid al-Aqsa.
Hadith and Sunnah about Rukn al-Yamani:
It is narrated from Ibn Umar that the Prophet ﷺ said:
“Touching the Black stone and Yamani Corner indeed erases sins.” (Sahih al-Jami: 2194)
Nafi’ (Allah be pleased with him) reported:
“I saw ‘lbn ‘Umar (Allah be pleased with them) touching the Stone with his hand and then kissing his hand and he said: I have never abandoned it since I saw Allah’s Messenger ﷺ doing It.” (Sahih Muslim: 1268)
Ibn ‘Umar (Allah be pleased with them) said:
“The Apostle of Allaah(ﷺ) did not give up touching the Yamani corner and the (Black) Stone in each of his circumambulations. Ibn ‘Umar used to do so.” (Sunan Abi Dawood: 1876)
Whenever Holy Prophet ﷺ circumbulate the Holy Kabaah and pass between the two corners, rukn al-Yamani and Hajar-e-Aswad, He recited: رَبَّنَا آتِنَا فِىْ الدُّنْيَا حَسَنَةً وَفِىْ الآخِرَةِ حَسَنَةً وَّقِنَا عَذَابَ النَّارِ
It is the Sunnah of Holy prophet ﷺ to do Istilaam on Rukn al-Yamani. But if there is massive crowd so it is not compulsory for the pilgrims to do Istilaam, they can skip. Rukun al-Yamani is the crack in the holy Kabaah which is impossible to mend. Many rulers tried many times to seal that crack but unable to do, so they poured a special type of concrete, molten metal to prevent it from disintegration.