It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When my Companions are mentioned, hold back; when the stars are mentioned, hold back; and when divine decree (qadar) is mentioned, hold back."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1414