Musnad Ahmad 974
It was narrated that ‘Abd Khair said:
`Ali bin Abi Talib (رضي الله عنه) came out to us when we were in the mosque and said: Where is the one who was asking about Witr? Those among us who had started the first rak`ah added a second to it so as to make it even, then we gathered around him and he said: The Messenger of Allah (ﷺ) used to pray Witr at the beginning of the night, then he prayed Witr in the middle of the night, then he settled on praying Witr at this time. He said: That was when dawn was breaking.
حَدَّثَنَا غَسَّانُ بْنُ الرَّبِيعِ، حَدَّثَنَا أَبُو إِسْرَائِيلَ، عَنِ السُّدِّيِّ، عَنْ عَبْدِ خَيْرٍ، قَالَ خَرَجَ عَلَيْنَا عَلِيُّ بْنُ أَبِي طَالِبٍ رَضِيَ اللَّهُ عَنْهُ وَنَحْنُ فِي الْمَسْجِدِ فَقَالَ أَيْنَ السَّائِلُ عَنْ الْوَتْرِ فَمَنْ كَانَ مِنَّا فِي رَكْعَةٍ شَفَعَ إِلَيْهَا أُخْرَى حَتَّى اجْتَمَعْنَا إِلَيْهِ فَقَالَ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يُوتِرُ فِي أَوَّلِ اللَّيْلِ ثُمَّ أَوْتَرَ فِي وَسَطِهِ ثُمَّ أَثْبَتَ الْوَتْرَ فِي هَذِهِ السَّاعَةِ قَالَ وَذَلِكَ عِنْدَ طُلُوعِ الْفَجْرِ‏.‏