It is narrated from Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever performs ablution and performs it well, then enters the Mosque of Quba and prays four rak'ahs in it, that is equal to freeing a slave."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5539