Muwatta Malik 1041

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 ‏.‏

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

Reference

Muwatta Malik 1041

In-book

Book 23, Hadith 1