It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The month is not always complete at thirty nights."
Musa bin Harun said: Its meaning is that not every month is complete at thirty; that is, sometimes it is twenty-nine.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 7005