Musnad Ahmad Hadith 21566

It is narrated from Abu Dharr (may Allah be pleased with him) that once, on the 23rd night of Ramadan, we stood in prayer with the Noble Prophet (ﷺ) until a third of the night had passed. Then the Noble Prophet (ﷺ) said, "I think that what you are seeking is still ahead (Laylat al-Qadr)." In the same way, on the 25th night, we stood in prayer until half the night had passed, then he said the same, "I think that what you are seeking is still ahead (Laylat al-Qadr)." Similarly, on the 27th night, we stood in prayer until morning, but this time the Noble Prophet (ﷺ) remained silent.