Musnad Ahmad 6:4

Asbat narrated to us, Mutarrif narrated to us, from ‘Amir, from Yahya ibn Talhah, from his father, who said: ‘Umar saw Talhah ibn ‘Ubaydullah looking heavy [distressed]. He said: "What is the matter with you, O Abu Fulan? Perhaps the leadership of your cousin has distressed you, O Abu Fulan?" He said: "No. It is only that I heard a hadith from the Messenger of Allah (peace and blessings of Allah be upon him), and nothing prevented me from asking him about it except the ability to do so, until he died. I heard him say: 'I surely know a word that no servant says at the time of his death, except that his complexion brightens because of it and Allah relieves him of his distress.'" He said: ‘Umar said: "I surely know what it is." He said: "And what is it?" He said: "Do you know of a word greater than the word he commanded his uncle to say at the time of death: *La ilaha illa Allah* (There is no god but Allah)?" Talhah said: "You have spoken the truth. It is, by Allah, it."