Musnad Abu Ya'la al-Mawsili 109:32

Ishaq narrated to us, he said: I heard Mu'tamir saying: My father narrated from Khidash, from Abu al-Zubayr, from Jabir ibn Abdullah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you lies on his back, he should not place one of his legs upon the other."