Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) was sitting in the courtyard of his house in Makkah when Uthman bin Maz'un (may Allah be pleased with him) passed by him, and he smiled at the Messenger of Allah (peace and blessings be upon him). The Messenger of Allah (peace and blessings be upon him) said to him: "Will you not sit?" He said: Yes, certainly. So the Messenger of Allah (peace and blessings be upon him) sat facing him. While they were still talking, the gaze of the Messenger of Allah (peace and blessings be upon him) rose towards the sky; he (peace and blessings be upon him) looked at the sky for a while, then fixed his gaze on a spot on the ground to his right. The Messenger of Allah (peace and blessings be upon him) turned away from his companion Uthman towards the place where he (peace and blessings be upon him) had fixed his gaze, and began nodding his head as though he (peace and blessings be upon him) were trying to understand what was being said to him, while Ibn Maz'un was watching. When he (peace and blessings be upon him) had finished his need and understood what had been said to him (peace and blessings be upon him), the Messenger of Allah (peace and blessings be upon him) raised his gaze (towards the sky) until he (the angel) disappeared into the sky. Then he (peace and blessings be upon him) turned back to Uthman in his first sitting position, and Uthman said: O Muhammad! Why was I sitting with you? I have never before seen you do as you did this morning. He (peace and blessings be upon him) said: "Did you notice that?" Uthman said: Yes. The Messenger of Allah (peace and blessings be upon him) said: "A messenger (angel) of Allah came to me while you were sitting." Uthman said: A messenger of Allah? He (peace and blessings be upon him) said: "Yes." Uthman said: Then what did he say to you? He (peace and blessings be upon him) said: "Indeed, Allah Almighty commands justice, excellence and giving (their right) to relatives, and forbids indecency, wrongdoing and transgression. He admonishes you so that you may take heed." [Surah al-Nahl: 90]. Uthman (may Allah be pleased with him) said: "That was the moment when faith settled in my heart and I came to love Muhammad (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8290