Reward of "La ilaha illallah" 1

Abu Nasr narrated that he had heard that Abul Hussain ibn Ahmad quote on the authority of his grandfather, on the authority of his father, “When Imam Reza (a) entered Nishapur at the time of Ma’moon, I was at his service and took care of his needs as long as he was there until he departed towards Sarakhs. I accompanied him when he wanted to leave Sarakhs to go to Marv. But as soon as we passed through one way-station, he brought out his head from his carriage and said, O Aba Abdullah! Return. May you be successful. You have performed what is incumbent upon you. You need not accompany me anymore. I again said, I swear by al-Mustafa (the Holy Prophet (s)), al-Murtaza (Imam Ali (a)) and al-Zahra (Hazrat Fatema (s)). Please tell me one tradition to heal me so that I may return. He (a) said, Are you asking me for traditions? I have left the neighborhood of my grandfather, the Prophet of Allah (s), not knowing what will happen to my affairs. I said, I swear by al-Mustafa, al-Murtaza and al-Zahra. Please tell me one tradition to heal me so that I may return. Then he (a) said, My father (a) narrated that his father (a) quoted on the authority of his father (a) that he had heard his forefathers (a) say that they heard their forefathers (a) say that they heard Ali ibn Abi Talib (a) say that he had heard the Holy Prophet (s) say that Allah, Mighty and Majestic be He said…