Musannaf Ibn Abi Shayba 13:792

Abu Bakr narrated to us, he said: Umar ibn Ayyub al-Mawsili narrated to us, from Ja'far, from Maymun, who said: If a man acknowledges a debt during his illness, then I am of the opinion that it is binding upon him, because if he had acknowledged it while he was healthy, it would have been binding. And the most truthful a person is, is at the time of his death.