Muwatta Imam Malik Book of Setting Free and Wala', Hadith 10
Malik related to me that he had heard that al-Maqburi said that Abu Hurayra was asked whether a man who had to free a slave, could free an illegitimate child to fulfil that obligation. Abu Hurayra said, "Yes. That will give satisfaction for him."
وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ عَنِ الْمَقْبُرِيِّ، أَنَّهُ قَالَ سُئِلَ أَبُو هُرَيْرَةَ عَنِ الرَّجُلِ، تَكُونُ عَلَيْهِ رَقَبَةٌ هَلْ يُعْتِقُ فِيهَا ابْنَ زِنًا فَقَالَ أَبُو هُرَيْرَةَ نَعَمْ ذَلِكَ يُجْزِئُ عَنْهُ .