A wall belonging to the Ansar, and he relieved himself there. Then he said to me: "O Abu Musa, guard the door for me, and let no one enter upon me except with permission." Then a man came and knocked at the door, and I said: "Who is this?" He said: "Abu Bakr." I said: "O Messenger of Allah, this is Abu Bakr seeking permission." He said: "Allow him in, and give him glad tidings of Paradise." So he entered, and I gave him glad tidings of Paradise. Then another man came and knocked at the door, and I said: "Who is this?" He said: "Umar." I said: "O Messenger of Allah, this is Umar seeking permission." He said: "Open for him, and give him glad tidings of Paradise." So I opened the door, and he entered, and I gave him glad tidings of Paradise. Then another man came and knocked at the door, and I said: "Who is this?" He said: "Uthman." I said: "O Messenger of Allah, this is Uthman seeking permission." He said: "Open for him, and give him glad tidings of Paradise, despite a trial that will befall him."(1)

This is a hasan sahih hadith, and it has been narrated through more than one chain from Abu Uthman al-Nahdi.
In this chapter, there is also a narration from Jabir and Ibn Umar.

4044 - Sufyan ibn Waki' narrated to us, saying: My father and Yahya ibn Sa'id narrated to us, from Isma'il ibn Abi Khalid, from Qays ibn Abi Hazim, who said:
Abu Sahlah narrated to me, saying: Uthman said on the Day of the House (i.e., the siege): Indeed, the Messenger of Allah--------------------------------------------
(1) A sahih hadith, narrated by al-Bukhari (3674), (3693), and (3695), Muslim (2407), al-Nasa'i in "al-Kubra" (8133), and it is in "al-Musnad" (19509) and (19643), and "Sahih Ibn Hibban" (6911) and (6912).