I heard the Messenger of Allah ﷺ as saying: Do not kill your children secretly, for the milk, with which a child is suckled while his mother is pregnant, overtakes the horseman and throws him from his horse.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ لاَ تَقْتُلُوا أَوْلاَدَكُمْ سِرًّا فَإِنَّ الْغَيْلَ يُدْرِكُ الْفَارِسَ فَيُدَعْثِرُهُ عَنْ فَرَسِهِ  

I intended to prohibit suckling during pregnancy (ghailah), but I considered the Greeks and the Persians and saw that they practiced it, without any injury being caused to their children thereby. Malik said : Ghailah means that a man has intercourse with a women while she is suckling a child.  

عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ عَنْ جُدَامَةَ الأَسَدِيَّةِ أَنَّهَا سَمِعَتْ رَسُولَ اللَّهِ ﷺ يَقُولُ لَقَدْ هَمَمْتُ أَنْ أَنْهَى عَنِ الْغَيْلَةِ حَتَّى ذُكِّرْتُ أَنَّ الرُّومَ وَفَارِسَ يَفْعَلُونَ ذَلِكَ فَلاَ يَضُرُّ أَوْلاَدَهُمْ قَالَ مَالِكٌ الْغَيْلَةُ أَنْ يَمَسَّ الرَّجُلُ امْرَأَتَهُ وَهِيَ تُرْضِعُ