It is narrated from Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Neither one suck nor two sucks (of suckling) make (marriage) unlawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15622