Musnad Ahmad 2:171

Yahya narrated to us, from Isma'il, [who] narrated to us 'Amir; and Muhammad ibn 'Ubayd narrated to us, [who] narrated to us Isma'il ibn Abi Khalid, from a man, from al-Sha'bi, who said: 'Umar passed by Talhah—and he mentioned its meaning—he said: 'Umar passed by Talhah and saw him looking concerned. He said: "Perhaps the leadership of your cousin has distressed you?"—he meant Abu Bakr. He said: "No, but I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'I surely know a word; no man says it at the time of his death except that it is a light in his record, or he finds relief for it at the time of death.'" 'Umar said: "I will inform you of it. It is the word he wanted for his uncle: the testimony that there is no god but Allah." He said: "It was as if a covering had been lifted from me." He said: "You have spoken the truth. If he had known a word better than it, he would have commanded him with it."