In the name of Allah, the Most Merciful, the Ever Merciful.
The messenger of Allah (saws) was such an amazing individual, it was impossible for him to say anything except the truth. He was asked by his wife Aisha (raa), "Which of your wives do you love best?" Now, if you know much about the life of the Prophet (saws), then you know how much he loved Aisha. He once said to his daughter Fatima (raa), "Don't you love what I love?" She responded, "Yes." He said, "Then love Aisha." He had an incredibly strong love for Aisha, and yet when she asked him, "Which of your wives do you love best?" what did he say (saws)? He said, "Khadijah." Who was Khadijah? Khadijah was his first wife. She was the one he was married to for years before he even became the messenger of Allah (saws). She was the one who comforted and consoled him on the night of his first revelation from the Divine, when he ran to her crying in a state of fear and shock, saying to her, "cover me up." She was the first to believe in him and accept Islam. So when Aisha (raa) asked him, "Which of your wives do you love best?" he (saws) said, "Khadijah." And Aisha (raa) said, Why do you love Khadijah better? She's long dead, and I'm younger..." and this and that, and the Prophet (saws) became upset, and he said, "Khadijah was with me when everybody had abandoned me. She gave me wealth and supported me…" Subhanallah, think about this. What normal man would say that to the wife that he was going to be living with now, about the previous wife. You'd except him to say, no I prefer you. But the Prophet (saws) was not a normal man. She asked him, and he told her the truth. This was the exceptional character of the messenger of Allah (saws).
"And indeed, you [O Muhammad] are of tremendous character" (Qur'an 68:4)