Narrated from Zirr with reference to Ubayy bin Ka'b (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) informed us regarding this matter that the night he mentioned is the twenty-seventh night of Ramadan, and its sign is that after this night passes, the next day when the sun rises, it appears as if it is circling and it has no rays.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21191
Hadith Gradingحکم دارالسلام:حديث صحيح، م: 792، وهذا إسناد حسن فى المتابعات والشواهد من أجل الأجلح لكنه توبع