Ibn Umar Radiyallahu Anhu reports that Rasulullah said,
“When you meet a haji (on his way home) then greet him, shake hands with him and ask him to beg forgiveness of Allah on your behalf before he enters his home, for his prayer for forgiveness is accepted since he is forgiven by Allah for his sins.”
In another hadith it is mentioned that Rasulullah once prayed,
“Oh Allah, forgive the sins of the hajis and forgive the sins of him for whom the haji prays.”
It is reported that Rasulullah made this dua three times which emphasises its importance.
For these reasons it has always been the custom and habit of the learned predecessors to welcome the pilgrims, to walk with them and to ask them to pray on ones behalf.
Taken from: ~Fadhail-e-Haj by Shaykh Zakariyya Kandehlawi Rahimahullah~
‘Do not look at how small the sin is but instead look at how great is the One that you have disobeyed.’