A man came to the Prophet ﷺ and said, "Prophet of Allah, I am an old, frail man, and standing in prayer is difficult for me. Tell me of a particular night in which Allah may grant me to coincide with Laylat al-Qadr."He replied, "Take to the seventh."
حدثنا معاذ بن هشام حدثني أبي عن قتادة عن عكرمة
أن رجلا أتى النبي ﷺ فقال يا نبي الله إني شيخ كبير عليل يشق علي القيام فأمرني بليلة لعل الله يوفقني فيها لليلة القدر قال عليك بالسابعة
Its chain of transmission is sahih. It appears that what is meant by "the seventh" is the seventh night remaining of Ramadan. Shawkani said (4:393), "Or the seventh night after the twentieth had passed."The hadith appears in Majma al-Zawaid (3:176), where it is stated, "Ahmad narrated it, and its narrators are the narrators of the Sahih." It is also found in Muntaqa (2294), and Shawkani also attributed it to Tabarani in al-Kabir. See also 2052.As for his saying, "He grants me success in it, Laylat al-Qadr," this is how it appears in both original manuscripts, and it has a grammatical justification in Arabic. In Majma al-Zawaid it reads, "for Laylat al-Qadr," with the addition of the preposition lam. See also 2302, 2352.