Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) relates: I heard the Prophet (peace and blessings be upon him) say: "One prayer offered in al-Masjid al-Haram is more excellent than a thousand prayers in any other mosque."
Imam al-Tabarani recorded this in al-Mu'jam al-Kabir. Its chain contains a narrator, Sahl bin Ubaydah al-Tustari, and I have not found anyone who has written a biography of him; the rest of the narrators of this narration are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5864