Musnad Ahmad #243

It was narrated that ash-Sha`bi said: `Umar passed by Talhah, and he narrated a similar report. He said: ‘Umar passed by Talhah and saw him looking worried. He said: Perhaps you are upset because your cousin was appointed (as caliph)? - referring to Abu Bakr. He said: No, but by Allah, I heard the Messenger of Allah (ﷺ) say: `I know a word which, if a man says it when he is dying, it will be light in the record of his deeds, or he will find comfort in it at the time of death.` ‘Umar said: I will tell you of it; it is the word that he wanted his paternal uncle to say, the testimony that there is no god but Allah. He said: It is as if a great burden has been lifted from me. He said: You are right. If he had known anything better that it, he would have instructed him to say it.