It is narrated from Sayyidah Hafsah (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) lay down on his bed, he would place his right hand under his right cheek; and his (peace and blessings be upon him) right hand was for eating, drinking, purification and water, and his left hand was for other tasks.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19651