The Quran

۞ تُرجي مَن تَشاءُ مِنهُنَّ وَتُؤوي إِلَيكَ مَن تَشاءُ ۖ وَمَنِ ابتَغَيتَ مِمَّن عَزَلتَ فَلا جُناحَ عَلَيكَ ۚ ذٰلِكَ أَدنىٰ أَن تَقَرَّ أَعيُنُهُنَّ وَلا يَحزَنَّ وَيَرضَينَ بِما آتَيتَهُنَّ كُلُّهُنَّ ۚ وَاللَّهُ يَعلَمُ ما في قُلوبِكُم ۚ وَكانَ اللَّهُ عَليمًا حَليمًا

Muhammad Asad

[know that] thou mayest put off for a time whichever of them thou pleasest, and mayest take unto thee whichever thou pleasest; and [that,] if thou seek out any from whom thou hast kept away [for a time], thou wilt incur no sin [thereby]:¹ this will make it more likely that their eyes are gladdened [whenever they see thee],² and that they do not grieve [whenever they are overlooked], and that all of them may find contentment in whatever thou hast to give them: for God [alone] knows what is in your hearts - and God is indeed all-knowing, forbearing.³

Thus, the Prophet was told that he need not observe a strict "rotation" in the conjugal attentions due to his wives, although he himself, impelled by an inborn sense of fairness, always endeavoured to give them a feeling of absolute equality.
I.e., by the inner certainty that whenever he turned to any of them, he did so on impulse, out of genuine affection, and not out of a sense of marital "obligation".
According to a hadith on the authority of 'A'ishah, recorded in the Musnad of Ibn Hanbal, the Prophet "used to divide his attentions equitably among his wives, and then would pray: 'O God! I am doing whatever is in my power: do not, then, blame me for [failing in] something which is in Thy power [alone], and not in mine!' thus alluding to his heart, and to loving some [of his wives] more than others."