Abdullah bin Masud reported,
I would rather swear by Allah nine times that the Messenger of Allah ﷺ was killed than swear once that he was not, for indeed Allah 
took him as a Prophet and made him a martyr.
Ahmad 3617
Sahihحدثنا أبو معاوية حدثنا الأعمش عن عبد الله بن مرة عن أبي الأحوص
عن عبد الله قال
لأن أحلف بالله تسعا إن رسول الله ﷺ قتل قتلا أحب إلي من أن أحلف واحدة وذلك بأن الله عز وجل اتخذه نبيا وجعله شهيدا
Its isnad is sahih. Abu Ahwas is Awf bin Malik bin Nadla.The hadith was narrated by Hakim, 3:58, from Abu Abbas Asamm, from Ahmad bin Abdul-Jabbar, from Abu Muawiya, with this isnad, and he said, "A sahih hadith according to the criterion of the two Shaykhs, though they did not include it." Dhahabi concurred with him.Ibn Kathir quoted it in Tarikh, 5:227, from the narration of Bayhaqi, from Hakim, with his isnad.