Musnad Ahmad 498:4

Harun ibn Ma'ruf narrated to us, Ibn Wahb informed me, Yazid ibn 'Iyad informed us, from 'Imran ibn Abi Anas, from 'Abdullah ibn Nafi' ibn al-'Amya', from al-Muttalib ibn Rabi'ah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The night prayer is two by two. And when one of you prays, let him recite the tashahhud in every two rak'ahs, then let him be persistent in supplication. Then, when he supplicates, let him be still, let him show misery, and let him show weakness. Whoever does not do that, then that is deficient, or like the deficient."