A useful note:
Abū al-Qāsim al-Suhaylī states in al-Rawḍ (1), in substance: It is inconceivable that the Prophet’s passing (Allah bless him and grant him peace) occurred on Monday, the twelfth of Rabīʿ al-Awwal in the eleventh year, for he stood at ʿArafāt during the Farewell Pilgrimage in year ten on a Friday; therefore the first of Dhū al-Ḥijjah that year fell on a Thursday. Whether one counts the intervening months as all complete, all deficient, or some complete and some deficient, Monday can never coincide with the twelfth of Rabīʿ al-Awwal.

This objection to that view is well known, and many have attempted to answer it. The only viable response is to invoke the difference in moon-sighting: the people of Mecca saw the crescent of Dhū al-Ḥijjah on the night before Thursday, whereas the people of Madinah did not see it until the night before Friday. This is supported by the statement of ʿĀʾishah and others: “The Messenger of Allah (Allah bless him and grant him peace) set out from Madinah for the Farewell Pilgrimage with five days remaining of Dhū al-Qaʿdah.” From what we have stated, this means he departed on a Saturday—not, as Ibn Ḥazm claimed, on a Thursday, for unquestionably more than five days would then have remained. Nor could he have left on a Friday, because Anas (2) reported: “The Messenger of Allah (Allah bless him and grant him peace) prayed the noon prayer in Madinah as four rakʿahs and the afternoon prayer at Dhū al-Ḥulayfah as two.” It is therefore certain that he left on Saturday with five days remaining. Thus the people of Madinah did not sight the crescent of Dhū al-Ḥijjah until the night before Friday. If Friday was the first of Dhū al-Ḥijjah for them, and the months thereafter are counted as full, the first of Rabīʿ al-Awwal will be a Thursday, making its twelfth day a Monday. Allah knows best.

It is established in the two Ṣaḥīḥs (3) from the ḥadīth of Mālik, from Rabīʿah ibn Abī ʿAbd al-Raḥmān, from Anas ibn Mālik, who said:

“The Messenger of Allah (Allah bless him and grant him peace) was neither strikingly tall nor short, neither extremely pale nor dark. His hair was neither tightly curly nor completely straight. Allah commissioned him as a Prophet at the age of forty; he remained in Mecca for ten years and in Madinah for ten years. Allah took him to Himself at the age of sixty, and there were not twenty white hairs in his head and beard.”

Ibn Wahb likewise transmitted it from Qurrah (4) from al-Zuhrī, from Anas, and from Qurrah through (5) Rabīʿah, from Anas, with the same wording.

Al-Ḥāfiẓ Ibn ʿAsākir said: “The report of Qurrah from al-Zuhrī is unusual, but from Rabīʿah through Anas it has been narrated by several transmitters in this form.” He then cited—by way of Sulaymān ibn Bilāl (6)—from Yaḥyā ibn Saʿīd and Rabīʿah, from Anas: “The Messenger of Allah (Allah bless him and grant him peace) passed away at the age of sixty-three.”