Zirr says that Ubayy ibn Ka'b (may Allah be pleased with him) told me that Laylat al-Qadr is on the twenty-seventh night of Ramadan. By the sign which the Noble Prophet (peace and blessings be upon him) told us, we calculated and counted it, and it was that very night.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21196
Hadith Gradingحکم دارالسلام:حديث صحيح، م: 762، وهذا إسناد حسن