Musnad Ahmad 25:311

Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatadah, from 'Ikrimah, from 'Abdullah ibn 'Abbas, that a man came to the Prophet, may Allah bless him and grant him peace, and said: "O Prophet of Allah, I am an old, sick man, and standing is difficult for me, so command me to a night, perhaps Allah will grant me success in it to [attain] Laylat al-Qadr." He said: "You must observe the seventh [night]."