The Prophet (peace and blessings be upon him) said: "I should certainly not find any of you reclining on his couch when a matter of mine comes to him, something I have commanded or something I have forbidden, and he says: I will only follow what we have found in the Book of Allah."
Imam al-Shafi'i (may Allah have mercy on him) said: So he commanded following what he commanded and avoiding what he forbade, and Allah made that obligatory upon His creation in His Book. What people have of this is only what they hold to from Allah, then from the Messenger (peace and blessings be upon him), then from what his guidance indicates. But his saying, if he said it, "Let people not hold me to anything," indicates that the Prophet (peace and blessings be upon him), since he was in the position of the one to be followed, had particular matters in which things were permitted to him that were not permitted to people, and things were forbidden to him that were not forbidden to people. So he said: Let people not hold me to anything of what is for me or upon me to the exclusion of them; if it is among what is upon me and for me to the exclusion of them, let them not hold to it. That is, for example, that Allah, exalted be His praise, made lawful for him whatever number of women he wished, and that he may marry a woman who gives herself to him, and Allah Almighty said: "exclusively for you, not for the (rest of the) believers" [Surah al-Ahzab: 50]. So no one may say: The Messenger of Allah (peace and blessings be upon him) combined more than four (wives), married a woman without a dowry, took a chosen share (safi) from the spoils, and had a fifth of the fifth, so this is for the believers after him, or for their rulers, as it was for him; because Allah has made clear in His Book and on the tongue of His Messenger (peace and blessings be upon him) that this was for him to the exclusion of them. And Allah made it obligatory upon him to give his wives the choice between staying with him and separating, so no one may say: It is obligatory upon me to give my wife a choice, according to what Allah made obligatory upon His Messenger (peace and blessings be upon him). This is the meaning of the saying of the Prophet (peace and blessings be upon him), if he said it: "Let people not hold me to anything."
The Shaykh (al-Bayhaqi) said: Al-Shafi'i (may Allah have mercy on him) only hesitated about the authenticity of the report, saying "if he said it", because the hadith is mursal and there is nothing with it to support it; but if it is understood as al-Shafi'i (may Allah have mercy on him) explained, it is clear and in agreement with the principles.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13441