Sayyidah Su'da al-Murriyyah (may Allah be pleased with her) said: After the death of the Messenger of Allah (peace and blessings be upon him), Sayyiduna Umar (may Allah be pleased with him) passed by Sayyiduna Talhah (may Allah be pleased with him) while he was grieved, and said to him: What is the matter with you? Did the leadership of your cousin displease you? He said: No, but I had heard the Messenger of Allah (peace and blessings be upon him) say: "I know a word which, if a person says it at the time of his death, it will become a light in his record of deeds, and his body and soul will find comfort in it at the time of death." And I never asked him ((peace and blessings be upon him)) about that word until he ((peace and blessings be upon him)) passed away. At this Umar (may Allah be pleased with him) said: I know that word; it is the very word to which he ((peace and blessings be upon him)) called his uncle (Abu Talib) at the time of his death, and had he ((peace and blessings be upon him)) known anything better than it that could have saved him, he ((peace and blessings be upon him)) would certainly have commanded him to say it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21142