Muwatta Malik 1051

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَزِيدَ بْنِ عَبْدِ اللَّهِ بْنِ قُسَيْطٍ اللَّيْثِيِّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ كَانَ يَقُولُ ذَكَاةُ مَا فِي بَطْنِ الذَّبِيحَةِ فِي ذَكَاةِ أُمِّهِ إِذَا كَانَ قَدْ تَمَّ خَلْقُهُ وَنَبَتَ شَعَرُهُ ‏.‏

Yahya related to me from Malik from Yazid ibn Abdullah ibn Qusayt al-Laythi that Said ibn al- Musayyab said, "The slaughter of what is in the womb is included in the slaughter of the mother if it is perfectly formed and its hair has begun to grow

Reference

Muwatta Malik 1051

In-book

Book 24, Hadith 10