These four quotations from historical sources should help answer this question.
Al-Hakim, in his al-Mustadrak, states that the Prophet kept confiding in 'Ali till the time of his death. Then he breathed his last.
When the body of the Prophet (s) was being given ghusl, according to Tarikh al-Khamis, al-’Abbas, Fadhl and Qutham turned the body of the Prophet (s) from one side to the other as Usamah and Shaqran poured water over it. All of them were blind-folded.
Ibn Sa'd narrates in his al-Tabaqat from Imam 'Ali that the Prophet had so enjoined that if anyone except himself ('Ali) had given him the funeral bath, he would have gone blind.
Ibn ‘Abd al-Barr, in his al-Isti'ab, quotes 'Abdullah ibn 'Abbas as saying that 'Ali had four such exceptional honours to his credit as none of us had, and he listed them all, the fourth being that he was the only person who gave the Prophet his funeral bath and lowered him in his grave.