Mukhtasar Sahih Bukhari — Hadith 931

Narrated by Sayyiduna Ibn Umar (may Allah be pleased with them both) from the Noble Prophet (peace and blessings of Allah be upon him) that he said: "We are an unlettered people; we do not know calculation. (Then, by gesturing with his fingers, he explained that) the month is like this and sometimes like this, meaning sometimes it is twenty-nine days and sometimes thirty days."