Tawus Yamani narrates that Imam Zainul Abideen (a.s) was seen during the Hajj season rubbing his cheeks on ground near the black stone and reciting devotions to his Lord.
“O my God! Your slave has come to Your House. Your impoverished one has come to Your House. Your beggar has come to Your House. Your pleader has come to Your House.”
The Imam used to say that three types of people worship in this world: The first type worship due to fear. It is the worship of slaves. The second type worship for rewards – It is the worship of business people. The third type worship with thankfulness – This is the true worship of the servants of Allah. He used to subject his body to utmost penance. One day Imam Baqir (a.s) asked him why he practiced such a severe penance? He replied, “Don’t you like that I should obtain divine proximity?”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams