Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: We were with the Messenger of Allah (peace and blessings be upon him) and the people mentioned Laylat al-Qadr, so the Messenger of Allah (peace and blessings be upon him) said: "How much of the month has passed?" They said: Twenty-two, and eight remain. He (peace and blessings be upon him) said: "Twenty-two have passed and seven remain. The month is twenty-nine (days). So seek it tonight." Then Abu Nu'aym said: Yes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8540