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
Sayyidah Aishah (may Allah be pleased with her) says that it was said to me: O Mother of the Believers, can the month of Ramadan be twenty-nine days? She said: I fasted twenty-nine days with the Messenger of Allah (peace and blessings be upon him) more often than thirty.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8201
Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: I fasted twenty-nine days with the Messenger of Allah (peace and blessings be upon him) more often than thirty.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8202
Sayyiduna Abu Bakrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Two months of Eid do not fall short: Ramadan and Dhu al-Hijjah."
The meaning of the hadith, and Allah knows best, is that even if they come out as twenty-nine days, they are complete with respect to the rulings connected with them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8203