In which source does Imam Mahdi say "Kitab Al-Kafi is enough for our Shia"?

Bismillahir Rahmanir Rahim. Alhamdulillahi rabbil-'alamin. Was-Salatu was-Salam 'ala Sayyidina wa Azimina, Wa Habibi Qulubina wa Shafii Nufusina, Abul Qasim Muhammad. Wa 'ala Ahli Bayti tayyibina tahireen. ---------- According to a Shia scholar Imam Mahdi, before he went into Ghaiba, had said "Kitab Al-Kafi is enough for our Shia". I heard it from a video in YouTube without the source being mentioned. So the question is, **In which source does Imam Mahdi say "Kitab Al-Kafi is enough for our Shia"?**