Sahih Muslim 1267
Sahih Muslim · Book 15: The Book of Pilgrimage · It is recommended to touch the two Yemeni Corners in Tawaf and not the other two corners
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى ، حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ ، عَنْ عُبَيْدِ اللَّهِ ، عَنْ نَافِعٍ ، عَنْ عَبْدِ اللَّهِ ، ذَكَرَ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ لاَ يَسْتَلِمُ إِلاَّ الْحَجَرَ وَالرُّكْنَ الْيَمَانِيَ.
Nafi' reported on the authority of 'Abdullah (Allah be pleased with him) that Allah's Messenger (way peace be upon him) did not touch but the Stone and the Yamani corner.