Imam lived for a year in Baghdad with Ummul Fadhl. She was very disobedient to Imam. When she found out that Imam had another wife (from the progeny of Ammar-e-Yasir) and that there was also children she was jealous and angry realizing that her father's plan had failed.
She complained to her father who also realized that his plan, to keep the 12th Imam in his progeny, had failed. He was enraged and in his rage he drank heavily and went to the 9th Imam's house and attacked Imam with a sword. Both Ummul Fadhl and a servant saw the attack and believed Imam was dead. Mamun on waking next morning realized the consequences of his attack and was thinking of arranging the disposal of Imam's body when he saw Imam well without a scratch on him.
He was confused and asked Imam who showed him an amulet which is called Hirze Jawad Imam told him it was from his grand mother Bibi Fatima Zahra (A.S.) and kept the wearer safe from all except the angel of death. Mamun asked Imam for it and Imam gave him one.
source : alhassanain
