Muwatta Malik, Book 30, Hadith 1286 (In-Book) ⓘ

Yahya related to me from Malik from Ibn Shihab that he said, "Suckling however little or much, makes haram. Kinship by suckling makes men mahram." Yahya said that he had heard Malik say, "Suckling, however little or much when it is in the first two years, makes haram. As for what is after the first two years, little or much, it does not make anything haram. It is like food."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ كَانَ يَقُولُ الرَّضَاعَةُ قَلِيلُهَا وَكَثِيرُهَا تُحَرِّمُ وَالرَّضَاعَةُ مِنْ قِبَلِ الرِّجَالِ تُحَرِّمُ ‏.‏ قَالَ يَحْيَى وَسَمِعْتُ مَالِكًا يَقُولُ الرَّضَاعَةُ قَلِيلُهَا وَكَثِيرُهَا إِذَا كَانَ فِي الْحَوْلَيْنِ تُحَرِّمُ فَأَمَّا مَا كَانَ بَعْدَ الْحَوْلَيْنِ فَإِنَّ قَلِيلَهُ وَكَثِيرَهُ لاَ يُحَرِّمُ شَيْئًا وَإِنَّمَا هُوَ بِمَنْزِلَةِ الطَّعَامِ ‏.‏