It is mentioned in the lengthy story about Sa’d ibn Hishaam ibn ‘Aamir, when he came to Madeenah and went to ‘Aa’ishah (may Allaah be pleased with her) and asked her about some matters. He said: I said: O Mother of the believers, tell me about the character of the Messenger of Allaah (S). She said: Do you not read the Qur’aan? I said: Of course. She said: The character of the Prophet of Allaah (S) was the Qur’aan. I wanted to get up and not ask about anything else until I died… Narrated by Muslim (746).
According to another report:
I said: O Mother of the believers, tell me about the character of the Messenger of Allaah (peace and blessings of Allaah be upon him). She said: O my son, do you not read the Qur’aan? Allaah says (interpretation of the meaning): “And verily, you (O Muhammad صلى الله عليه وسلم) are on an exalted (standard of) character” [al-Qalam 68:4]. The character of Muhammad was the Qur’aan.
Narrated by Abu Ya’la (8/275) with a saheeh isnaad.
Al-Nawawi (may Allaah have mercy on him) said in Sharh Muslim (3/268):
What this means is that he acted in accordance with it, adhering to its limits, following its etiquette, paying heed to its lessons and parables, pondering its meanings and reciting it properly. End quote.
Ibn Rajab said in Jaami’ al-‘Uloom wa’l-Hukam (1/148):
What this means is that he followed its etiquette and adopted its attitude. Whatever was praised in the Qur'aan, he was pleased with, and whatever was condemned in the Qur'aan he hated. It says in one report that she said: His attitude was the Qur’aan, whatever it was pleased with he was pleased with and whatever it hated he hated. End quote.
Al-Munaawi said in Fayd al-Qadeer (5/170):
i.e., whatever the Qur’aan contained of commands, prohibitions, promises, warnings, and so on.
Al-Qaadi said: i.e., his attitude embodied everything that was contained in the Qur’aan. Whatever it regarded as good, praised or promoted was his attitude, and whatever it regarded as bad and prohibited, he would avoid. So the Qur’aan described his character. End quote.