It is narrated from Yahya bin Sa'id (may Allah have mercy on him) that Abu Musa (may Allah be pleased with him) said about the nursing of an adult: I can only consider that it establishes prohibition. Ibn Mas'ud (may Allah be pleased with him) said: Consider carefully the ruling you are giving the man. Abu Musa (may Allah be pleased with him) said: Then what do you say? Ibn Mas'ud (may Allah be pleased with him) said: Fosterage is only what takes place within the two years. Abu Musa (may Allah be pleased with him) said: Do not ask me (about anything) as long as this learned scholar is among you.
Although this is mursal, it has supporting narrations (shawahid) from Ibn Mas'ud (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15664