Masruq (may Allah have mercy on him) said: I came to Madinah and asked about the Companions of the Messenger (peace and blessings be upon him), and they told me that Zayd bin Thabit (may Allah be pleased with him) was among those firmly grounded in knowledge.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12198