Hadithcore
Mishkat al-Masabih, 4155

Ibn ‘Abbas told that God’s messenger sacrificed a ram on the seventh day for both al-Hasan and al-Husain. Abu Dawud transmitted it. Nasa’i says two rams each.

وَعَنِ ابْنِ عَبَّاسٍ: أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَقَّ عَنِ الْحَسَنِ وَالْحُسَيْنِ كَبْشًا كَبْشًا. رَوَاهُ أَبُو دَاوُدَ وَعِنْدَ النَّسَائِيِّ: كبشين كبشين

References1 variant
In-Book Reference
Book 20, Hadith 90