Musnad Ahmad 612:6

He said: "O Ka'b ibn Murrah, narrate to us from the Messenger of Allah, may Allah bless him and grant him peace, and be cautious." He said: "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Whoever frees a Muslim person, it will be his ransom from the Fire; for every bone of his, a bone of his will be recompensed. And whoever frees two Muslim women, they will be his ransom from the Fire; for every two bones of theirs, a bone of his will be recompensed. And whoever grows a grey hair in the way of Allah, it will be a light for him on the Day of Resurrection.'"