Muwatta Imam Malik Book of Tahajjud, Hadith 22

Yahya related to me from Malik from Abdullah ibn Dinar that Abdullah ibn Umar used to say, "The maghrib prayer is the witr of the daytime prayers."
Malik said, "If someone prays witr at the beginning of the night, and goes to sleep, and then wakes up and it seems good to him to pray, let him pray, two rakas at a time. That is what I like most of what I have heard."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ صَلاَةُالمغرب وتر صلاة النهار . قال مالك من أوتر أول الليل ثم نام ثم قام فبدا له أن يصلي فليصل مثنى مثنى فهو أحب ما سمعت إلى