Sayidina Uthman (r.a) prayed 4 rakahs instead of 2 during the Haj. This was enough reason for some hypocrites to accuse him.
But everything has own explanation.
al-Bayhaqi narrated in “al-Marifah” as a taliq, and declared its chain of transmission good: From Abdurrahman ibn Humayd from his father that Uthman ibn Affan completed prayer at Mina and then delivered the speech to people. He said: “People! The Sunnah is the Sunnah of the Messenger of Allah (sallalahu alaihi wa ala alihi wa sallam)! And the Sunnah of his two companions. However, this year are many new people and I was afraid that they will take it as a sunnah”.
Note: Mean he has prayed 4 rakahs during the journey instead of 2, because he afraid that in case he will pray two, new comers will continue in this way even not during the journeys.
at-Tahawi and Abu Dawud narrated from az-Zuhri, and chain is strong mursal in accordance to allama Nimawi, that he said: “Uthman only prayed four at Mina because the desert Arabs were more numerous that year and he preferred to teach them that the prayer is four rakahs”.
Both reports taken from Athar as-Sunnah by allama Nimawi Hanafi pp 371-372.