Musnad Ahmad 5:210

Abu Abd al-Rahman narrated to us, Musa ibn Ayyub narrated to us, my uncle Iyas ibn Amir narrated to me, I heard Ali ibn Abi Talib saying: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray voluntary prayers (tasbih) at night, while Aisha was lying across between him and the qibla."