Sunan an-Nasa'i, Volume 5, Book 43, Hadith 4389 (DarusSalam) ⓘ

It was narrated that 'Asim bin Kulaib said: "I heard my father narrating from a man who said: 'We were with the Messenger of Allah two days before Al-Adha and we started to offer two Jadh ahs for one Thaniiyah, Then the Messenger of Allah said: 'A Jadh'ah is sufficient for that for which a Thaniyah is sufficient."

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ عَاصِمِ بْنِ كُلَيْبٍ، قَالَ سَمِعْتُ أَبِي يُحَدِّثُ، عَنْ رَجُلٍ، قَالَ كُنَّا مَعَ النَّبِيِّ صلى الله عليه وسلم قَبْلَ الأَضْحَى بِيَوْمَيْنِ نُعْطِي الْجَذَعَتَيْنِ بِالثَّنِيَّةِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ الْجَذَعَةَ تُجْزِئُ مَا تُجْزِئُ مِنْهُ الثَّنِيَّةُ ‏" ‏ ‏.‏