Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) reports that the Prophet (peace and blessings be upon him) said: "Whoever I am the mawla of, Ali is also his mawla."
Al-Bazzar recorded it within a longer hadith, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14632