Sahih Muslim
Sahih Muslim 1267 a
It is recommended to touch the two Yemeni Corners in Tawaf and not the other two corners
Arabic hadith text
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا اللَّيْثُ، ح وَحَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا لَيْثٌ، عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ قَالَلَمْ أَرَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَمْسَحُ مِنَ الْبَيْتِ إِلاَّ الرُّكْنَيْنِ الْيَمَانِيَيْنِ .
Translation
Ahdullah b. Umar (reported) that he had not seen Allah's Messenger (way peace he upon him) touching anything in the House, except the two Yamani corners.