Musnad Ahmad 978
It was narrated from Mujalid that ‘Amir said:
Sharahah had a husband who was absent in Syria. She became pregnant and her former master brought her to `Ali bin Abi Talib (رضي الله عنه) and said: This one has committed zina, She admitted it, so he gave her one hundred lashes on Thursday and stoned her on Friday; he dug a hole for her to her navel, and I was present. Then he said: Stoning is a Sunnah established by the Messenger of Allah  (ﷺ). If anyone saw her do it, the first one to throw a stone should be the one who witnessed it; he should give his testimony and follow his testimony with his stone. But she admitted it, so I will be the first one to stone her. He threw a stone at her, then the people stoned her and I was among them. By Allah, I was among those who killed her.
حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ مُجَالِدٍ، حَدَّثَنَا عَامِرٌ، قَالَ كَانَ لِشَرَاحَةَ زَوْجٌ غَائِبٌ بِالشَّامِ وَإِنَّهَا حَمَلَتْ فَجَاءَ بِهَا مَوْلَاهَا إِلَى عَلِيِّ بْنِ أَبِي طَالِبٍ رَضِيَ اللَّهُ عَنْهُ فَقَالَ إِنَّ هَذِهِ زَنَتْ فَاعْتَرَفَتْ فَجَلَدَهَا يَوْمَ الْخَمِيسِ مِائَةً وَرَجَمَهَا يَوْمَ الْجُمُعَةِ وَحَفَرَ لَهَا إِلَى السُّرَّةِ وَأَنَا شَاهِدٌ ثُمَّ قَالَ إِنَّ الرَّجْمَ سُنَّةٌ سَنَّهَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَلَوْ كَانَ شَهِدَ عَلَى هَذِهِ أَحَدٌ لَكَانَ أَوَّلَ مَنْ يَرْمِي الشَّاهِدُ يَشْهَدُ ثُمَّ يُتْبِعُ شَهَادَتَهُ حَجَرَهُ وَلَكِنَّهَا أَقَرَّتْ فَأَنَا أَوَّلُ مَنْ رَمَاهَا فَرَمَاهَا بِحَجَرٍ ثُمَّ رَمَى النَّاسُ وَأَنَا فِيهِمْ قَالَ فَكُنْتُ وَاللَّهِ فِيمَنْ قَتَلَهَا‏.‏