Musnad Ahmad 32:313

Yazid ibn Harun narrated to us, Al-Awwam ibn Hawshab informed us, from Sulayman ibn Abi Sulayman, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, who said: "When Allah, the Almighty and Majestic, created the earth, it began to sway. So He created the mountains and cast them upon it, and it became stable. The angels were amazed at the creation of the mountains and said: 'O Lord, is there anything in Your creation stronger than the mountains?' He said: 'Yes, iron.' They said: 'O Lord, is there anything in Your creation stronger than iron?' He said: 'Yes, fire.' They said: 'O Lord, is there anything in Your creation stronger than fire?' He said: 'Yes, water.' They said: 'O Lord, is there anything in Your creation stronger than water?' He said: 'Yes, the wind.' They said: 'O Lord, is there anything in Your creation stronger than the wind?' He said: 'Yes, the son of Adam who gives charity with his right hand, concealing it from his left.'"