Sunan Abi Dawud Book 5, Hadith 1
The Prophet (ﷺ) as saying: If anyone prays in a day and a night twelve rak'ahs voluntarily (supererogatory prayer), a house will be built from him in Paradise on account of these (rak'ahs).
The Messenger of Allah (ﷺ) sent me to Khalid b. Sufyan al-Hudhail. This was towards 'Uranah and 'Arafat. He (the Prophet) said: Go and kill him. I saw him when the time of the afternoon prayer had come. I said: I am afraid if a fight takes place between me and him (Khalid b. Sufyan), that might delay the prayer. I proceeded walking towards him while I was praying by making a sign. When I reached near him, he said to me: Who are you ? I replied: A man from the Arabs; it came to me that you were gathering (any army) for this man (i.e. Prophet). Hence I came to you in connection with this matter. He said: I am (engaged) in this (work). I then walked along with him for a while ; when it became convenient for me, I dominated him with my sword until he became cold (dead).
I asked A'ishah about the voluntary prayers offered by the Messenger of Allah (ﷺ). She replied: Before the noon prayer he would pray four rak'ahs in my house, then go out and lead the people in prayer, then return to my house and pray two rak'ahs. He would lead the people in the sunset prayer, then return to my house and pray two rak'ahs. Then he would lead the people in the night prayer, and enter my house and pray two rak'ahs. He would pray nine rak'ahs during the night, including witr (prayer). At night he would pray for a long time standing and for a long time sitting. When he recited the Qur'an while standing, he would bow and prostrate himself from the standing position, and when he recited while sitting, he would bow and prostrate himself from the sitting position, and when dawn came he prayed two rak'ahs, then he would come out and lead the people in the dawn prayer.