Musnad Ahmad 949:5

Zayd ibn al-Hubab narrated to us, Hammad—meaning ibn Salamah—narrated to me, from Sa'id ibn Jumhan, Safinah Abu 'Abd al-Rahman narrated to me, he said: I heard the Prophet, may Allah bless him and grant him peace, saying: "The Caliphate will be thirty years, then it will be kingship." Then he mentioned it.