Sayyiduna Buraydah (may Allah be pleased with him) narrates this saying of the Prophet (peace and blessings be upon him): "The aqiqah (animal) should be slaughtered on the seventh day, or on the fourteenth day, or on the twenty-first day."
Al-Tabarani recorded it in al-Saghir and al-Awsat; its chain contains Isma'il bin Muslim al-Makki, who is da'if (weak) because of his many errors and misconceptions.