Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that Sayyiduna Umar (may Allah be pleased with him) called the Companions of the Prophet (peace and blessings be upon him) and asked them about Laylat al-Qadr, and they all agreed that it is in the last ten. I said to Sayyiduna Umar (may Allah be pleased with him): I think I know which night it is. He asked: Which night is it? I said: It is when seven have passed, or seven remain, of the last ten. He asked: How do you know? I said: Allah Almighty created seven heavens, seven earths and seven days, and time too revolves in sevens; man was created, and he eats and prostrates on seven limbs; the circuits (tawaf) are seven, and the mountains are seven. Sayyiduna Umar (may Allah be pleased with him) said: You have grasped what we had not grasped.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8558