Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) used to encourage the night prayer, to the extent that he would say: "Even if it is a single rak'ah." Then once he went out for the prayer and saw a man praying while the obligatory prayer was being established, so the Messenger of Allah (peace and blessings be upon him) said: "Two prayers together?"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11479