Hadithcore
Hadith #510440

Yahya related to me from Malik from Nafi that Abdullah ibn Umar did not sacrifice for the child in the womb. Malik said, "The sacrifice is sunna, and it is not obligatory. I prefer that anyone who has the price of the animal should not abandon it.''

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، لَمْ يَكُنْ يُضَحِّي عَمَّا فِي بَطْنِ الْمَرْأَةِ ‏.‏ قَالَ مَالِكٌ الضَّحِيَّةُ سُنَّةٌ وَلَيْسَتْ بِوَاجِبَةٍ وَلاَ أُحِبُّ لأَحَدٍ مِمَّنْ قَوِيَ عَلَى ثَمَنِهَا أَنْ يَتْرُكَهَا ‏.‏

References1 variant
Arabic/English book reference
Book 23, Hadith 13