Musannaf Ibn Abi Shayba 9:894

Abu Bakr narrated to us, from Waki’, from ‘Amr ibn Marwan, from ‘Abd al-Rahman al-Suda’i, from ‘Ali, who said: A man came to him and said: "I have a cousin whom I desire, and I had attained something from her." He said: "If it was an internal thing," meaning sexual intercourse, "then no. But if it was an external thing," meaning kissing, "then there is no harm."