Sahih Bukhari Volume 002, Book 023, Hadith Number 372.
Narated By Anas bin Malik : The Prophet passed by a woman who was weeping beside a grave. He told her to fear Allah and be patient. She said to him, "Go away, for you have not been afflicted with a calamity like mine." And she did not recognize him. Then she was informed that he was the Prophet. so she went to the house of the Prophet and there she did not find any guard. Then she said to him, "I did not recognize you." He said, "Verily, the patience is at the first stroke of a calamity."