Sahih Muslim 10540
In-book no 3263وَحَدَّثَنَاهُ أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ الأَعْلَى، عَنْ مَعْمَرٍ، عَنِ الزُّهْرِيِّ، بِهَذَا الإِسْنَادِ غَيْرَ أَنَّهُ قَالَ " تُشَدُّ الرِّحَالُ إِلَى ثَلاَثَةِ مَسَاجِدَ " .
This hadith has been narrated on the authority of Zuhri (but with this change of words) that he (Allah's Apostle) said: " Undertake journey to three mosques."