Bulugh al-Maram, Book 12, Hadith 1357 (In-Book)

'Aishah (RAA) narrated, "Allah's Messenger commanded them to slaughter two sheep equal (in age) for a boy and one for a girl at their birth.' Related by At-Tirmidhi and graded it as Sahih.

وَعَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا; { أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-أَمْرَهُمْ; أَنْ يُعَقَّ عَنْ اَلْغُلَامِ شَاتَانِ مُكَافِئَتَانِ, وَعَنْ اَلْجَارِيَةِ شَاةٌ } رَوَاهُ اَلتِّرْمِذِيُّ وَصَحَّحَه ُ (1780)‏.‏ ‏3 ‏- صحيح.‏ رواه الترمذي ( 1513 )‏، وقال: "حديث حسن صحيح".‏

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also referenced under: Book 12, Hadith 1398 (English)