It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Sometimes the month is such, such, and such," and the third time the Prophet (peace and blessings be upon him) folded his thumb, "and sometimes it is such, such, and such," meaning a complete thirty days.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5484
Hadith Gradingحکم دارالسلام:إسناده صحيح ، م :1080.