Anas bin Malik narrated that:
The Prophet said: "When Allah created the earth, it started shaking. So He created the mountains, and said to them: 'Upon it' so it began to settle. The angels were amazed at the strength of the mountains, so they said: 'O Lord! Is there among your creatures one who is more severe than the mountains?' He said: 'Yes. Iron.' They said: 'O Lord! Then is there anything among your creatures that is more severe than that iron?' He said: 'Yes. Fire.' S they said: 'O Lord! Is there anything among your creatures that is more severe than fire?' He said: 'Yes. Water.' They said: 'O Lord! Is there anything among your creatures that is more severe than water?' He said: 'Yes. Wind.' They said: 'O Lord! Is there anything among your creatures more severe than wind?' He said: 'Yes. The son of Adam. He gives charity with his right hands, while hiding it from his left.'"
English reference: Vol. 5, Book 44, Hadith 3369
Arabic reference: Book 47, Hadith 3695