When leaving the الكعبة for the last time, exit with the left foot and say: 'Bismillah, was-salatu was-salamu 'ala Rasulillah. Allahumma inni as'aluka min fadlik. Allahumma la taj'alhu akhiral-'ahdi bi baytik' — O Allah, do not let this be my last time at Your House. Ask Allah to bring you back again.
بِسْمِ اللَّهِ وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُولِ اللَّهِ. اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ. اللَّهُمَّ لَا تَجْعَلْهُ آخِرَ الْعَهْدِ بِبَيْتِكَ، اللَّهُمَّ ارْزُقْنِي الْعَوْدَ إِلَيْهِ بِخَيْرٍ وَعَافِيَةٍ
Bismillah, was-salatu was-salamu 'ala Rasulillah. Allahumma inni as'aluka min fadlik. Allahumma la taj'alhu akhiral-'ahdi bi baytik, Allahumma-rzuqnil-'awda ilayhi bi khayrin wa 'afiyah.
Meaning: In the name of Allah, and peace and blessings be upon the Messenger of Allah. O Allah, I ask You of Your bounty. O Allah, do not let this be my last visit to Your House. O Allah, grant me return to it in goodness and well-being.
Source: Sahih Muslim #713This farewell moment is deeply emotional for الحجاج, as they do not know if they will ever return to the House of Allah. The دعاء specifically asks Allah not to make this the final visit, expressing the الحاج's longing to return. Many scholars مستحب looking at the الكعبة one last time and making whatever personal دعاء comes from the heart, as it may be the last opportunity.
بِسْمِ اللَّهِ وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُولِ اللَّهِ. اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ. اللَّهُمَّ لَا تَجْعَلْهُ آخِرَ الْعَهْدِ بِبَيْتِكَ، اللَّهُمَّ ارْزُقْنِي الْعَوْدَ إِلَيْهِ بِخَيْرٍ وَعَافِيَةٍ
The Prophet (peace be upon him) said: 'When one of you leaves the masjid, let him say: Allahumma inni as'aluka min fadlik.'