It is narrated from al-Mughirah bin Shu'bah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Al-fiqah does not make (marriage) unlawful." We said: O Messenger of Allah, what is al-fiqah? He (peace and blessings be upon him) said: "A woman gives birth and milk gathers in her breasts, then she suckles her neighbour's (child) once or twice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15636