Narrated Qatada:

Anas bin Malik said, "The Prophet () used to visit all his wives in a round, during the day and night and they were eleven in number." I asked Anas, "Had the Prophet () the strength for it?" Anas replied, "We used to say that the Prophet () was given the strength of thirty (men)." And Sa`id said on the authority of Qatada that Anas had told him about nine wives only (not eleven).

حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، قَالَ حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، قَالَ حَدَّثَنَا أَنَسُ بْنُ مَالِكٍ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم يَدُورُ عَلَى نِسَائِهِ فِي السَّاعَةِ الْوَاحِدَةِ مِنَ اللَّيْلِ وَالنَّهَارِ، وَهُنَّ إِحْدَى عَشْرَةَ‏.‏ قَالَ قُلْتُ لأَنَسٍ أَوَكَانَ يُطِيقُهُ قَالَ كُنَّا نَتَحَدَّثُ أَنَّهُ أُعْطِيَ قُوَّةَ ثَلاَثِينَ‏.‏
وَقَالَ سَعِيدٌ عَنْ قَتَادَةَ إِنَّ أَنَسًا حَدَّثَهُمْ تِسْعُ نِسْوَةٍ‏.‏

Reference	: Sahih al-Bukhari 268
In-book reference	: Book 5, Hadith 21