Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "When one of you is lying on his back, let him not place one of his feet over the other."
Al-Bazzar recorded it, and its narrators are the narrators of the Sahih, except Khidash al-Abdi, and he is thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 13185