Mishkat al-Masabih 693
Abu Sa'id al-Khudri reported God’s Messenger as saying, “Animals are saddled only for prayer in the three mosques:
the sacred mosque, the Aqsa mosque, and this mosque of mine.”* 

* These are the only mosques to which long journeys may be made purely for the purpose of praying in them. The sacred mosque means the Ka'ba.

(Bukhari and Muslim.)
وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: 
" لَا تُشَدُّ الرِّحَالُ إِلَّا إِلَى ثَلَاثَةِ مَسَاجِدَ: مَسْجِدِ الْحَرَامِ وَالْمَسْجِدِ الْأَقْصَى وَمَسْجِدِي هَذَا "