Umar asked, "Who knows when Laylat al-Qadr is?"Ibn Abbas replied, "The Messenger of Allah ﷺ said, 'It is in the last ten nights, after seven have passed or with seven remaining.'"
حدثنا عفان حدثنا عبد الواحد بن زياد حدثنا عاصم الأحول
قال عمر من يعلم متى ليلة القدر قالا فقال ابن عباس قال رسول الله ﷺ هي في العشر في سبع يمضين أو سبع يبقين
Its isnad is sahih. Lahiq bin Humayd Sadusi was a trustworthy successor (tabii) who heard from Ibn Umar, Ibn Abbas, and Anas bin Malik, and Bukhari mentioned him in al-Kabir, 4/2/258-259. He and Ikrima did not meet Umar, but the hadith is the hadith of Ibn Abbas, so it appears that it was Ibn Abbas who related to them Umar's question and his answer to it. See 2520.