Musnad Abi Ya'la 823 — the Messenger of Allah ﷺ said

It is narrated from Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said (indicating with the fingers of his hand): "The month is like this, and like this, and like this." And the third time he held back one finger (meaning that the month is sometimes also of twenty-nine days).