Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "We are an unlettered nation; we neither write nor calculate. The month is like this and like this, meaning thirty days." Then he (peace and blessings be upon him) said: "Like this, like this, like this," and he folded his thumb, meaning twenty-nine. He (peace and blessings be upon him) said twenty-nine on one occasion and thirty on another.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8200