Musnad Ahmad 788:159

Yahya ibn Sa'id narrated to us, from Uthman ibn Ghiyath, Abu Uthman narrated to us, from Abu Musa, that he was with the Prophet, may Allah bless him and grant him peace, in a garden, and in the hand of the Prophet, may Allah bless him and grant him peace, was a stick with which he was striking between the water and the mud. Then a man came asking to be let in. He said: "Open for him and give him glad tidings of Paradise." So it was Abu Bakr, may Allah be pleased with him. He said: So I opened for him and gave him glad tidings of Paradise. Then a man came asking to be let in. He said: "Open for him and give him glad tidings of Paradise." So it was Umar, may Allah be pleased with him. So I opened for him and gave him glad tidings of Paradise. Then a man came asking to be let in. He said: "Open for him and give him glad tidings of Paradise upon a trial that will befall him, or a trial that will occur." He said: So it was Uthman, may Allah be pleased with him. So I opened for him and gave him glad tidings of Paradise and informed him. He said: "Allah is the One sought for help."