مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3850
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said: To swear nine times that the Messenger of Allah (ﷺ) was killed is dearer to me than swearing once that he was not killed. That is because Allah made him a Prophet and took him as a martyr. Al-A'mash said: I mentioned that to Ibraheem and he said: They used to think that the Jews had poisoned him and Abu Bakr.
Musnad Ahmad 3873lts isnad is saheeh]